The blast that killed two people in Poland on Monday likely came from the remains of a Russian missile shot down by Ukraine, a Polish journalist tweeted.

"My sources in the services say that what hit Przewodowo is most likely the remains of a rocket shot down by the armed forces of Ukraine," according to a translation of a tweet by Mariusz Gierszewski, a writer who is covering the story for Polish Radio ZET.

The Associated Press quoted a senior U.S. intelligence official as saying the blast was due to Russian missiles crossing into Poland.

However, the Pentagon said on Tuesday it could not confirm reports that Russian missiles had crossed into Poland.

"Firefighters are on the spot, it's not clear what has happened," said Lukasz Kucy, officer on duty at a nearby firefighters' post.

Radio ZET reported earlier that two stray missiles hit Przewodow on Tuesday, killing two people, without giving any more details.

Latvian Deputy Prime Minister Artis Pabriks sent condolences to Poland in a tweet.

"Criminal Russian regime fired missiles which target not only Ukrainian civilians but also landed on NATO territory in Poland," he wrote. "Latvia fully stands with Polish friends and condemns this crime."

Russia was pounding cities across Ukraine with missiles on Tuesday, in attacks that Kyiv said were the heaviest wave of missile strikes in nearly nine months of war. Some hit Lviv, which is less than 50 miles from the border with Poland.

Polish Prime Minister Mateusz Morawiecki has called an urgent meeting of a government committee for national security and defense affairs, government spokesman Piotr Muller said on Twitter.

A government security spokesman and a representative of the prime minister's office declined to comment further.

State-run news agency PAP reported that the Polish government would hold a meeting at 2000 GMT.

Muller told reporters later on Tuesday that the committee was convened due to the "crisis situation," adding that relevant information will be presented to the public later. He called on media not to publish "unverified information" in the meantime. 

Information from Reuters was used in this report.