"The missile strikes in Poland last night are being fully investigated by our intelligence service, which is in close contact with NATO partner services," Ludivine Dedonder said in a statement.

"Based on the preliminary information available, the strikes are most likely a result of Ukrainian anti-aircraft systems that were engaged to take Russian missiles out of the sky."

According to Dedonder, "pieces of Russian missiles and Ukrainian anti-aircraft fire hit Polish territory... I wish to express my condolences to the families of the victims and the Polish people".

Russia launched a wave of missile strikes against Ukraine on Tuesday, but insists it was not to blame for the blast on Polish territory that triggered fears the conflict could spread.

NATO powers meeting in Brussels on Wednesday have pledged full solidarity with their fellow member state Poland, while condemning Moscow's latest attacks on Western-backed Ukraine.

However Western leaders, including US President Joe Biden, have been cautious about describing the explosion in Poland.

"It's unlikely... that it was fired from Russia. But we'll see," Biden said.