Rishi Sunak said the UK would “support our allies” in a bid to find out exactly what happened, following reports that Russian missiles crossed into Poland on Tuesday.
It comes as US President Joe Biden held a call with Polish President Andrzej Duda, in which Mr Biden “offered full US support for and assistance with Poland’s investigation”.
The Prime Minister, who is currently at the G20 in Bali with other world leaders, said he had spoken to Foreign Secretary James Cleverly and Defence Secretary Ben Wallace about the incident.
Mr Sunak also spoke himself to Mr Duda and “reiterated the UK’s solidarity” with the country and “expressed condolences for the victims and their families”.
He tweeted: “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 coordinating with our international partners, including NATO.”
According to Downing Street, Mr Sunak offered Poland “any assistance needed to urgently establish what happened”.
“The leaders agreed to remain in close contact and continue co-ordinating with our international partners, including Nato allies, on the next steps,” a spokesperson said.
Mr Cleverly earlier said that the UK was “urgently looking” into the reports.
Little information is so far known about the incident, but it has already sparked concern in Ukraine and among the war-torn nation’s allies in Europe and beyond.
The Polish Foreign Ministry late on Tuesday said that a Russian-made missile fell in eastern Poland and killed two people.
The ministry said foreign minister Zbigniew Rau summoned the Russian ambassador and “demanded immediate detailed explanations”.
Mr Biden, in his call with the Polish president, “offered full US support for and assistance with Poland’s investigation” and “reaffirmed the United States’ ironclad commitment to Nato”.
Mr Biden also spoke to Nato secretary general Jens Stoltenberg, who was among those to hold a call with Mr Duda.
Mr Stoltenberg said Nato was “monitoring the situation”, tweeting: “Allies are closely consulting. Important that all facts are established.”
It came as Ukrainian President Volodymyr Zelensky decried the missile strike as “a very significant escalation” of his country’s war with Russia.
In his nightly address, the Ukrainian leader said the reported strikes offered proof that “terror is not limited by our state borders”.
“We need to put the terrorist in its place. The longer Russia feels impunity, the more threats there will be for everyone within the reach of Russian missiles,” Mr Zelensky said.
European leaders were among those expressing deep concern about the development, amid urgent efforts to determine the exact circumstances behind the incident.
European Commission president Ursula von der Leyen was among those registering her alarm.
She tweeted: “Alarmed by reports of an explosion in Poland, following a massive Russian missile strike on Ukrainian cities.
“I extend my condolences and my strongest message of support and solidarity with Poland and our Ukrainian friends.”
