The US, EU and UK said the world will be watching to ensure Russia’s words are followed by action after the UN and Turkey helped to broker the deal.
International aid agencies and diplomats expect grain to start fully flowing by the middle of August, but market analysts said the situation will not change quickly.
“We fully expect the implementation of today's arrangement to commence swiftly to prevent the world's most vulnerable from sliding into deeper insecurity and malnutrition,” White House spokesman John Kirby said.
“We're hopeful that this is going to make a difference. But we're clear-eyed about it.”
UK Foreign Secretary Liz Truss, one of the two Conservative leaders competing to replace outgoing prime minister Boris Johnson, said London “will be watching to ensure Russia's actions match its words”.
She praised the efforts of Turkey and the UN, saying Russian President Vladimir Putin's “barbaric invasion of Ukraine has meant some of the poorest and most vulnerable people in the world are at risk of having nothing to eat”.
“It is vital that Ukrainian grain reaches international food markets, and we applaud Turkey and the UN secretary general for their efforts to broker this agreement,” she said. “The UK and our allies have been pushing hard to reach this point.
“Now this agreement must be implemented, and we will be watching to ensure Russia's actions match its words. To enable a lasting return to global security and economic stability, Putin must end the war and withdraw from Ukraine.”
The EU also called for the deal's “swift implementation” as European Commission President Ursula von der Leyen praised the team that brokered the deal.
“Russia's illegal invasion of Ukraine has left millions at risk of hunger. Today's Istanbul agreement is a step in the right direction. We call for its swift implementation,” EU foreign policy chief Josep Borrell said.
UN Secretary General Antonio Guterres called it “a beacon of hope” for millions of hungry people who have faced huge increases in food costs.
“Today, there is a beacon on the Black Sea. A beacon of hope …, possibility … and relief in a world that needs it more than ever,” he said.
US wheat futures fell more than 5 per cent on Friday to their lowest level since February, traders said.
Dr Stephen Flynn, professor of political science and director of the Global Resilience Institute at Northeastern University in Boston, said that while the deal is important, it “will not change things overnight in terms of grains availability around the world’s markets”.
“The challenges … are huge,” he said. “The backlog of 22 million tons to be cleared is only the first part … the complexity of being able to move shipping in and out of areas like with minefields and with inspection agreements is not going to be nimble.”
Two African presidents who met on Friday both had bigger hopes for the deal.
“I’m pleased to note that President Putin has decided to sign this proposal made by Turkey and the UN which will allow 20 million tons of Ukrainian wheat to be released,” said Ivory Coast's President Alassane Ouattara.
“I indicated before to President (Volodymyr) Zelenskyy that I wished for the supply to be prioritised towards the African continent because of the fragility of the economies and the social situation in many countries.”
Cyril Ramaphosa, President of South Africa, said the agreement had taken far too long to reach because “that conflict has put a stop to the importation or exportation of grain, fertilisers and other foodstuffs like wheat to various other parts of the world.
“Would this be seen as signalling something that could amount to the end of that conflict? I would like to believe that, yes, we should hopefully see it in that regard,” he said.