Ukrainian experts are already working at the site of the explosion in Poland's Przewodow, Ukrainian Foreign Minister Dmytro Kuleba said on Friday.

"I am grateful to the Polish side for granting them access. We will continue our cooperation in an open and constructive manner, as closest friends do," he tweeted. 

On Tuesday, two Polish citizens were killed by the projectile falling in the Polish village of Przewodow near the Poland-Ukraine border.

Warsaw and its Western allies say evidence from the scene points to the explosion being caused by a Ukrainian air defence missile that went astray in pursuit of a Russian missile. 

But Kyiv has denied this, saying it has evidence of a "Russian trace" in the blast.

(With input from agencies)