NATO ambassadors will meet on Wednesday at the request of Poland on basis of the alliance's Article 4, two European diplomats told Reuters after an explosion in Poland close to the Ukrainian border reportedly caused by a stray Russian missile.

According to article 4 of the alliance's founding treaty, members can raise any issue of concern, especially related to the security of a member country.

One of the diplomats said the alliance would act cautiously and needed time to verify how exactly the incident happened.