A rocket that appeared to be Russian-made fell on a Polish village and killed two people, Poland said on Tuesday.
It is the first time that a Nato country has been struck by a missile since Russia's invasion of its neighbour Ukraine in February.
Russia's Defence Ministry denied reports that missiles from the country had hit Polish territory, describing the reports as a deliberate provocation.
The missile landed in Poland at 2.20pm on Tuesday and killed two people in the village of Przewodow, Polish Foreign Ministry spokesman Lukasz Jasina said.
Polish President Andrzej Duda said he had no evidence of who fired the missile that caused the explosion.
“We do not have any conclusive evidence at the moment as to who launched this missile … it was most likely a Russian-made missile, but this is all still under investigation at the moment,” Mr Duda said.
After the explosion, Polish Prime Minister Mateusz Morawiecki urged his countrymen not to panic.
“I am calling on all Poles to remain calm in the face of this tragedy … we must exercise restraint and caution,” Mr Morawiecki said after emergency government meetings in Warsaw.
US President Joe Biden and leaders of key allies held “emergency” talks on Wednesday following the missile strike near Poland's border with Ukraine, the White House said.
Leaders of the EU and all G7 countries — Britain, Canada, France, Germany, Italy, Japan and the US — were among those at the hastily arranged meeting in Bali, where they have been taking part in a G20 summit of major economies.
Mr Biden told reporters there was “preliminary information” that contests the notion that the missile was fired from Russia.
“We're going to collectively determine our next steps as we investigate and proceed,” Mr Biden said.
He said there was “total unanimity” among the world leaders present.
The US President added that next steps for the international coalition would include a meeting of ambassadors.
Poland is putting some military units on a heightened state of readiness, government spokesman Piotr Muller said.
“There has been a decision to raise the state of readiness of some combat units and other uniformed services,” Mr Muller said.
He did not mention the reports of missiles but said there had been an explosion in eastern Poland in which two citizens were killed.
“Our services are on the ground at the moment working out what happened,” Mr Muller said after an emergency security council meeting in Warsaw.
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It is the first time missiles have been reported to have hit a Nato member state since Russia invaded Ukraine on February 24.
The Pentagon earlier said it could not immediately corroborate the reports and was investigating further. The US National Security Council said it was working with the Polish government to gather more information.
“We cannot confirm the reports or any of the details at this time. We will determine what happened and what the appropriate next steps would be,” said spokeswoman Adrienne Watson.
Mr Biden spoke by phone with Mr Duda from Bali.
“President Biden offered full US support for and assistance with Poland’s investigation,” the White House said after the call.
Mr Biden later tweeted: “I spoke with President Andrzej Duda of Poland to express my deep condolences for the loss of life in Eastern Poland and offer our full support for Poland's investigation of the explosion.”
Meanwhile, Nato Secretary General Jen Stoltenberg said on Twitter: “Spoke with President Duda about the explosion in Poland. I offered my condolences for the loss of life.
“Nato is monitoring the situation and allies are closely consulting. Important that all facts are established.”
Nato ambassadors will meet on Wednesday at the request of Poland on the basis of the alliance's Article 4, two European diplomats told Reuters.
Mr Duda said it was very likely that Poland would activate Nato's Article 4 at the meeting.
Article 4 of the alliance's founding treaty says members can raise any issue of concern, especially related to the security of a member country.
One of the diplomats said the alliance would act cautiously and needed time to verify exactly how the incident happened.
The situation will also be discussed at a UN Security Council session on Wednesday afternoon.
Ukraine's President Volodymyr Zelenskyy said that the strike was a “significant escalation” of the conflict.
“Russian missiles hit Poland,” Mr Zelenskyy said in a text accompanying his nightly video address. He did not provide evidence of the strikes.
“The longer Russia feels impunity, the more threats there will be to anyone within reach of Russian missiles.
“To fire missiles at Nato territory! This is a Russian missile attack on collective security. This is a very significant escalation. We must act.”
Mr Zelenskyy said he had spoken by phone with Mr Duda.
“Expressed condolences over the death of Polish citizens from Russian missile terror,” he tweeted.
“We exchanged available information and are clarifying all the facts. Ukraine, Poland, all of Europe and the world must be fully protected from terrorist Russia.”
Ukraine's Defence Minister Oleksiy Reznikov, meanwhile, said in response to the reports: “This is the reality we've been warning about. We were asking to close the sky because sky has no borders … gloves are off. Time to win.”
I want this Russian war to end, Zelenskyy urges world leaders at G20 - video
I want this Russian war to end, Zelenskyy urges world leaders at G20
European Council President Charles Michel said on Twitter: “Just spoke with [Polish Prime Minister] Mateusz Morawiecki. Assured him of full EU unity and solidarity in support of Poland.
“I will propose a co-ordination meeting on Wednesday with EU leaders attending G20 here in Bali.”
Latvia and Estonia have condemned the strikes.
Tallinn is “ready to defend every inch” of Nato territory, the Estonian Ministry of Foreign Affairs said on Twitter.
UK Foreign Secretary James Cleverly has said officials are “urgently looking into reports of missiles landing in Poland” and were in contact with Nato allies.
Late on Tuesday, UK Prime Minister Rishi Sunak spoke to Mr Duda and “reiterated solidarity with Poland as a close ally and expressed condolences for the victims and their families”, Downing Street said.
Mr Sunak said on Twitter: “We are urgently looking into reports of a missile strike in Poland and will support our allies as they establish what has happened.
“We are also co-ordinating with our international partners, including Nato.”
French President Emmanuel Macron asked for verification work to be carried out and German Foreign Minister Annalena Baerbock said: “My thoughts are with Poland. We are monitoring the situation closely. We are in contact with our Polish friends and Nato allies.”
Hungary's Prime Minister Viktor Orban convened the national defence council.
“In response to the stop in oil transfer through the Druzhba pipeline and the missile hitting territory of Poland, Prime Minister Viktor Orban has convened Hungary's defence council for 8pm,” tweeted Mr Orban's spokesman, Zoltan Kovacs.
Putin vows continued 'tough' attacks against Ukraine — video
Putin vows continued 'tough' attacks against Ukraine
Czech Prime Minister Petr Fiala said on Twitter that if Poland confirmed that Russian missiles hit its territory, it would be further escalation by Russia, and “we stand firmly behind our EU and Nato ally”.
Lithuanian President Gitanas Nauseda said his country stood in strong solidarity with Poland and that “every inch of Nato territory must be defended”.
German Chancellor Olaf Scholz assured the Polish President of his country’s solidarity.
“The Chancellor has just telephoned Polish President Duda and expressed his condolences,” Mr Scholz's spokesman tweeted.
“Poland will closely investigate the circumstances of the incident in which two citizens were killed last night. Germany stands closely by the side of our Nato partner Poland.”
The reports of missiles hitting Poland are “serious” and make it very important to establish what has happened, said Dutch Prime Minister Mark Rutte.
Belgium strongly condemned the incident, Belgian Prime Minister Alexander De Croo said, adding his country would stand with Poland.
“We are all part of the Nato family,” Mr De Croo tweeted.
Ukraine dismissed as a “conspiracy theory” Russian claims that the missile was Ukrainian.
“Russia now promotes a conspiracy theory that it was allegedly a missile of Ukrainian air defence that fell on the territory of Poland,” Ukrainian Foreign Minister Dmytro Kuleba said on Twitter.
“This is not true. No one should buy Russian propaganda or amplify its messages.”
