The Associated Press is reporting that the deadly explosion in Poland may have been caused by a missile fired by Ukrainian forces at an incoming Russian missile.

The news agency cites US officials as saying this is based on initial findings. Earlier on Wednesday, world leaders meeting in Bali for the G20 summit pledged support for Warsaw's investigation.

Asked whether the missile was fired from Russia, US President Joe Biden said, "There is preliminary information that contests that. I don't want to say that until we completely investigate. But it is unlikely in the minds of the trajectory that it was fired from Russia. But we will see, we'll see."

Officials in a village near the Ukraine border are collecting evidence.

Poland says it has confirmed that the damage was caused by a Russian-made missile, but that there is still no concrete proof of who fired it.

Poland's President Andrzej Duda is trying to ease fears of further attacks, saying the incident appears to be a one-off.

Duda said, "First of all, I want to assure everyone that we work in a very calm way, very carefully, we do not make any rash decisions and I ask everyone to stay calm. Polish soldiers are at the scene, we have allies' support, and all the commitments towards us are in place."

Russian defense officials have denied all reports of a missile attack on Poland, caling them a "deliberate provocation."