Poland raised its military readiness after a rocket struck its territory, killing two, in what the country's President Andrzej Duda said was "most probably" a strike by a Russian-made missile.
Warsaw said the missile strike happened in the south eastern village of Przewodow, near the border with Ukraine, but did not have conclusive evidence of who fired it.
Here are some of the major reactions from other nations:
US President Joe Biden held an emergency roundtable with G7 allies on the sidelines of the G20 summit in Indonesia.
He said they would support Poland in figuring out "exactly what happened" and then determine their next steps.
"It is unlikely... that it was fired from Russia. But we'll see," Biden told reporters after the meeting.
Earlier, Biden called Duda. The two leaders "said that they and their teams should remain in close touch to determine appropriate next steps as the investigation proceeds," according to the White House.
The president of Ukraine, which has been fighting a Russian invasion since February, offered condolences to Duda in a call.
"We exchanged information and are clarifying all the facts," Volodymyr Zelensky tweeted.
Foreign Minister Dmytro Kuleba dismissed as a "conspiracy theory" that the missile may have been fired by Ukrainian air defences.
The defence ministry in Moscow dismissed reports that missiles fired by Russian forces had landed in Poland.
"Polish mass media and officials commit deliberate provocation to escalate (the) situation with their statement on alleged impact of 'Russian' rockets," it said in an online statement.
"Russian firepower has launched no strikes at the area."
German Chancellor Olaf Scholz expressed solidarity with Polish President Andrzej Duda.
"The chancellor has just telephoned Polish President Duda and expressed his condolences... Germany stands closely by the side of our Nato partner Poland," Scholz's spokesman tweeted.
Nato member Britain's Prime Minister Rishi Sunak called Duda to express "solidarity", Downing Street said.
"The Prime Minister offered any assistance needed to urgently establish what happened," it added.
French President Emmanuel Macron had called for talks at the G20 summit in Indonesia over the strike, his office told AFP before the White House roundtable announcement.
In France's National Assembly, State Secretary Sonia Backes delivered a message to deputies from Prime Minister Elisabeth Borne: "The government is following the situation on the ground in conjunction with our Polish allies very closely."
"In any case, Poland can count on France's solidarity."
UN chief Antonio Guterres said after the strike that it was "absolutely essential" to avoid an escalation in the Ukraine war.
"The Secretary-General is very concerned by the reports of a missile exploding on Polish territory," according to his spokesman Farhan Haq.
EU chief Charles Michel said he was "shocked" by reports of the strike.
"My condolences to the families," he tweeted.
"We stand with Poland. I am in contact with Polish authorities, members of the European Council and other allies."
Czech Prime Minister Petr Fiala tweeted that he was in contact with his Polish counterpart following the missile strike.
"We are ready to coordinate further steps with our Polish friends," he said.