Russia on Wednesday rejoined a deal to allow Ukrainian grain exports through the Black Sea after pulling out of the agreement on Saturday.
The revival of the arrangement could ease fears of global food insecurity, but Russian President Vladimir Putin warned Moscow could again pull out of the deal.
The decision by Russia came as Washington said it was "increasingly concerned" that Mr Putin could use nuclear weapons in its campaign in Ukraine.
Moscow had said on Saturday that it was temporarily pulling out of the grain deal, accusing Ukraine of using a safe shipping corridor established under the agreement to launch a drone assault on its Black Sea fleet.
Russia's Defence Ministry said it had now received sufficient guarantees from Kyiv that it would not use the maritime corridor to carry out attacks.
UN Secretary General Antonio Guterres welcomed Russia's decision to rejoin the agreement, which was brokered by the UN and Turkey in July and allows for joint inspections of ships.
Ukraine and Russia sign grains deal - in pictures
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UN Secretary General Antonio Guterres, Ukrainian Infrastructure Minister Oleksandr Kubrakov, Turkish President Recep Tayyip Erdogan and Turkish Defence Minister Hulusi Akar attend a signing ceremony in Istanbul, Turkey. Reuters -
Ukrainian Infrastructure Minister Oleksandr Kubrakov, seated, at the signing ceremony. Reuters -
UN Secretary General Antonio Guterres, left, Russia's Defence Minister Sergei Shoigu, second left, Turkish President Recep Tayyip Erdogan, seated, second right, and Turkish Defence Minister Hulusi Akar at the signing ceremony. Reuters -
Mr Guterres speaks during the signing ceremony. Reuters -
Mr Guterres and Mr Erdogan sit at the start of the signature ceremony for an agreement on the safe transportation of grain and foodstuffs from Ukrainian ports. AFP -
Mr Erdogan speaks at the signing ceremony. Reuters -
Mr Guterres and Turkish Foreign Minister Mevlut Cavusoglu stand together on the day of the deal signing in Istanbul. Reuters -
Roman Abramovich attends the ceremony in Istanbul. Reuters -
Mr Guterres said the deal would clear the way for grain shipments from three Ukrainian ports; Odesa, Chernomorsk and Yuzhny, stabilising runaway prices on the global market. Reuters -
The UN chief said a co-ordination centre would be set up in Istanbul to manage Black Sea traffic. Reuters -
A Turkish national flag, a Russian national flag, a United Nations flag and a Ukrainian national flag in Istanbul, before the deal was signed. AFP
Ukrainian President Volodymyr Zelenskyy said on Twitter that he had thanked Turkish President Recep Tayyip Erdogan for his role in preserving the grain deal.
Mr Zelenskyy hailed its resumption as "a significant diplomatic result for our country and the whole world".
But Mr Putin said Russia could leave the deal again if Ukraine breached its guarantees, although Moscow would not interfere with any grain deliveries even if it did.
The Kremlin had warned that the route was dangerous for shipments without its participation in the agreement but some deliveries from Ukraine still went ahead on Monday and Tuesday.
A Turkish security source said the corridor was open again from 9am GMT although no departures from Ukraine were planned on Wednesday.
Russia's Deputy Foreign Minister Andrei Rudenko said Moscow had yet to decide if it would remain part of the deal after November 18.
Ukraine war latest - in pictures
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Smoke rises on the outskirts of the city during a Russian missile attack in Kyiv. Reuters -
Workers surround the Monument to the Heroes of the Heavenly Hundred with sandbags against damage from shelling in Mykolaiv. EPA -
Russian President Vladimir Putin welcomes Azerbaijan President Ilham Aliyev for talks before a meeting between the Russian president and the leaders of Armenia and Azerbaijan in the Black Sea resort city of Sochi, Russia. AFP -
People take water from a water pump in Kyiv. EPA -
A woman walks past a damaged building in the town of Nova Kakhovka. Reuters -
A woman outside her home, which was destroyed during battles at the start of the conflict, in Yahidne, Chernihiv. Getty Images -
People shelter inside a subway station during a Russian missile attack in Kyiv. Reuters -
Firefighters at the site of a drone attack in Kyiv, Ukraine. AP -
A woman walks past the site of a Russian missile strike in the southern Ukrainian city of Mykolaiv. Reuters -
Rescuer workers at a building destroyed in an attack in Mykolaiv. Reuters -
Parts of a drone lie on a street in Kyiv. Reuters -
A drone flies over the Ukrainian capital during an attack. AFP -
Ukrainian servicemen tow a captured Russian armoured vehicle in Rudneve village, Kharkiv. EPA -
A partially destroyed residential building in Saltivka, in Kharkiv. AP -
Ukrainian servicemen fire a captured Russian howitzer on a front line near Kupyansk city, Kharkiv. EPA -
Ukrainian servicemen near the recently retaken town of Lyman in Donetsk region. AFP -
An officer from a Ukrainian national police emergency demining team prepares to detonate collected anti-tank mines and explosives near Lyman, in the Donetsk region. AFP -
A boy playing on a destroyed Russian tank on display in Kyiv. AFP -
A young couple hiding underground during an air alert in Zaporizhzhia. AFP -
Ukrainian firefighters looking for survivors after a strike in Zaporizhzhia. AFP -
Workers fix a banner reading 'Donetsk, Luhansk, Zaporizhzhia, Kherson - Russia!' to the State Historical Museum near Red Square in Moscow. AFP -
Residents try to cross a destroyed bridge in the Kharkiv region of Ukraine. AFP -
Alyona Kishinskaya helps to clean up a shop as it prepares to reopen in Balakiya, Ukraine, after a six-month Russian occupation. Getty Images -
Alla, 12, has a swinging time in Balakiya, Ukraine, as life goes on despite the war. Getty Images -
Ukrainian flags in the town square in Balakiya. Getty Images -
A destroyed Russian command centre in Izium, Ukraine. Getty Images -
In what may be a final farewell, young Russian recruits - escorted by their wives - walk to a train station in Volgograd before being sent to war in Ukraine. AP -
Russian recruits board the train to Ukraine in Volgograd. AP -
Ukrainian soliders drive a tank at the recently retaken eastern side of the Oskil River in Kharkiv region, Ukraine. AFP -
Volunteers pass boxes of food aid on a destroyed bridge over the Oskil River. AFP -
Ukrainian soliders rest on an armoured personnel carrier. AFP -
A sign warns of landmines in Izyum, eastern Ukraine. AFP -
This crater was left after a missile strike in the Donetsk region of Ukraine. AFP
The agreement comes up for renewal on November 19, but the extension is a separate issue and that decision will be made "taking into account all the accompanying factors", state news agency RIA Novosti reported Mr Rudenko as saying.
US State Department spokesman Ned Price on Wednesday called for the deal to be renewed, saying this "will ultimately inject even more predictability and stability in this marketplace and, most importantly, apply downward pressure to the prices" of global food.
The deal, overseen by the Joint Co-ordination Centre in Istanbul, has allowed more than 9.7 million tonnes of grain and other foodstuffs to leave Ukrainian ports.
This has brought much-needed relief to a global food crisis caused largely by the conflict between Russia and Ukraine, both major global grain exporters.
World grain prices, which had soared this week, began to ease on Wednesday after Russia announced it was returning to the deal, despite doubts over its future.
Three ships with grain leave Ukraine under landmark deal - video
Mr Putin had demanded "real guarantees", while Mr Zelenskyy on Tuesday urged "reliable and long-term protection" of the corridor.
The Russian Defence Ministry said it obtained written guarantees from Kyiv.
It said Ukraine guaranteed "the non-use of the humanitarian corridor and Ukrainian ports determined in the interests of the export of agricultural products for conducting military operations against the Russian Federation".
The White House said repeated discussion by Russian officials of the potential use of nuclear weapons in Ukraine has left US officials worried that a risk could become a reality.
"We have grown increasingly concerned about the potential as these months have gone on," said White House national security spokesman John Kirby.
Putin's inner circle – in pictures
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Head of Russia's SVR intelligence service Sergei Naryshkin has seen his status eroded by the strong resistance encountered in Ukraine, which he did not foresee. EPA -
Head of Russia's Roscosmos space agency Dmitry Rogozin has threatened to abandon a Nasa astronaut on the International Space Station. AFP -
Russian Defence Minister Sergei Shoigu was one of the architects of Moscow's intervention in Syria. AFP -
Russian President Vladimir Putin meets with Russia's oil giant Rosneft chief Igor Sechin. He has been described as Mr Putin's right-hand man. AFP -
Secretary of Russia's Security Council Nikolai Patrushev attends a meeting in Moscow. He is said to have known Mr Putin for 50 years. AFP -
Sergei Ivanov, Russian special representative on questions of ecology and transport, is a close friend of Mr Putin. AFP -
Federal Security Service director Alexander Bortnikov is responsible for a huge network of agents in Russia. AFP -
At 58, president of the Duma Lower House of Parliament Vyacheslav Volodin is one of the younger members of the inner circle. He has been touted as a possible successor to Mr Putin. AFP
Mr Kirby also said North Korea was sending a significant amount of artillery ammunition to Russia under cover of shipments to countries in the Middle East or North Africa.
He did not confirm a report in The New York Times that high-level Russian military officials recently discussed when and how they might use tactical nuclear weapons on the battlefield.
The report, which quoted unidentified US officials, said Mr Putin did not take part in the discussions and there was no indication that the Russian military had decided to use the weapons.
Russia's Foreign Ministry said the world's "top priority" should be to avoid a clash of nuclear powers.
"We are firmly convinced that in the current difficult and turbulent situation — a consequence of irresponsible and shameless actions aimed at undermining our national security — the top priority is to prevent any military clash of nuclear powers," the ministry said.
