The missile blast that killed two people appears to be an “unfortunate accident” and not an “intentional attack”, Poland’s president has said.
Andrzej Duda tweeted that officials had “no proof at the moment that it was a missile fired by the Russian side”.
He added: “There are many indications that it was an air defence missile, which unfortunately fell on Polish territory.”
NATO Secretary General Jens Stoltenberg said in a statement to media: “Yesterday’s explosion took place as Russia launched a massive wave of rocket attacks across Ukraine.
“An investigation into this incident is ongoing, and we need to await its outcome.
“But we have no indication this was the result of a deliberate attack and we have no indication Russia is preparing offensive military actions against NATO.
“Our preliminary analysis suggests that the incident was likely caused by Ukrainian air defence missile fired to defend Ukrainian territory against Russia cruise missile attacks.
“But let me be clear – this is not Ukraine’s fault. Russia bears ultimate responsibility as it continues its illegal war against Ukraine.”
Ukraine’s President Volodymyr Zelenskyy has said he has “no doubt” the missile “was not Ukrainian” and has called for his officials to have access to the site of the blast.
The preliminary findings came after Joe Biden and other western backers of Ukraine had thrown their weight behind the investigation, amid repeated assertions from Russia that it did not fire the missile.
The US President cast doubt on previous claims the missile, which hit a grain silo in Przewodow, was fired by Moscow.
“There is preliminary information that contests that.
“I don’t want to say that until we completely investigate it, but it is unlikely in the lines of the trajectory that it was fired from Russia, but we’ll see.”
Three US officials said preliminary assessments suggested it was fired by Ukrainian forces at an incoming Russian one.
Ukraine maintains stocks of Soviet and Russian-made weaponry, including air-defence missiles, and has also seized many more Russian weapons while beating back the Kremlin’s forces.
Ukrainian air defences worked furiously against the Russian assault on power generation and transmission facilities, including in Ukraine’s western region that borders Poland.
Ukraine’s military said 77 of more than 90 missiles fired were brought down, along with 11 drones.
Russia has insisted it did not fire the missile.
A spokesman for the defence ministry told Russia’s state-owned RIA news agency that its strikes on 15 November were no closer than 22 miles (35km) from the Polish border.
They claimed that images of the site showed the wreckage to be that of a Ukrainian S-300 missile.
Mr Zelenskyy said damage from Tuesday’s missile attacks was extensive and large areas were plunged into darkness.
He said about 10 million people lost power but tweeted overnight that eight million had been reconnected.
Previous Russian strikes had already destroyed an estimated 40% of the country’s energy infrastructure.