US President Joe Biden has said it is “unlikely” that a missile that killed two people in Poland on Tuesday was fired from Russia.
Russia has denied it was to blame for the missile that landed in Przewodow, on the Ukrainian border.
Poland said it was Russian-made, but US officials said initial findings indicated it was fired by Ukrainian air defences.
More than 90 Russian missiles were fired at Ukraine on Tuesday, Kyiv said.
Although the military said 77 were shot down, some of the missiles hit Lviv, not far from Ukraine’s western border with Poland.
During the Russian attacks, two Polish workers were killed in a blast at a farm building in Przewodow, 6km (4 miles) from the border.
Nato member Poland’s military was placed on high alert, and Nato ambassadors were due to hold an emergency meeting in Brussels on Wednesday morning to assess their response.
While Russia condemned as a “provocation” initial reports that it had fired the missile into Poland, President Biden told reporters at the G20 in Bali that “preliminary information” contested those reports.
In a statement, Western leaders at the Bali summit condemned Russia’s “barbaric missile attacks” on Ukraine’s cities and civilian infrastructure and offered their full support to neighbouring Poland.
Among those attending were President Biden, UK Prime Minister Rishi Sunak, French President Emmanuel Macron and EU Commission President Ursula von der Leyen.
Ukraine’s Foreign Minister Dmytro Kuleba said earlier that any claim that Ukraine was responsible for Polish deaths was a Russian “conspiracy theory” and that anyone amplifying the message was spreading “Russian propaganda”.
Polish President Andrzej Duda told reporters that it remained unclear how the blast had occurred and said investigators were evaluating all possibilities.
“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,” he said.
Images from the Polish farm showed what appeared to be a large crater and a farm trailer lying on its side, suggesting missile damage. Another image showed a fragment of a missile.
“We are verifying the circumstances in which we might invoke Article 4,” said Poland’s National Security Bureau chief, Jacek Siewiera, referring to the Nato treaty. Article 4 enables member states to consult on whether the territorial integrity, political independence or security of any member state is threatened.
Despite lending support to Ukraine since Russia’s invasion, Nato has been careful not to become too heavily involved in the conflict in order to prevent an escalation.
United Nations Secretary General António Guterres also said he was “very concerned” by the Poland explosion and called for a thorough investigation.
“It is absolutely essential to avoid escalating the war in Ukraine,” said his spokesman, Farhan Haq.
The BBC’s Paul Adams said there were a number of possible explanations for the incident but Russia had no interest in targeting Polish farms.
As Ukraine’s air defences had been working hard to bring down Russian missiles, it was possible that one of the missiles fired was knocked off course, he added.
Ukrainian President Volodymyr Zelensky said most of the rockets fired by Russian forces had been aimed at the country’s energy infrastructure.
Nato Secretary General Jens Stoltenberg said he had spoken to Polish President Andrzej Duda and the military alliance was “monitoring the situation”.
“Allies are closely consulting,” he said on Twitter. “Important that all facts are established.”