By Tim Lister, Tara John, Antonia Mortensen, Anna Chernova and Emmet Lyons, CNN

(CNN) — Poland will increase the combat readiness of some troops and is considering activating Article 4 of the NATO military alliance treaty, officials said, following the deaths of two people in an explosion near the Ukrainian border.

Authorities did not say what caused the blast, but Polish media reported earlier in the day that missiles or rockets had struck the area.

Poland is in touch with NATO partners and US counterparts about the incident, government spokesman Piotr Muller said, and teams of experts will work on all night to clarify details of what he called a "serious" situation. "Poland is increasing the combat readiness of the army and other services," he added.

It remains unclear where the projectiles came from, but they landed in the NATO member's territory roughly the same time as Russia launched its biggest wave of missile attacks on Ukrainian cities in more than a month.

At the site of the explosion in the Hrubieszów district of eastern Poland, local media showed an image of a deep impact and upturned farm vehicle, near the town of Przewodow, around four miles west from the Ukrainian border. CNN cannot independently confirm the photos.

Video taken by a resident, which was geolocated and confirmed by CNN, shows a large smoke plume in the center of the village.

A local resident told CNN they heard a terrifying "whoosh" as the projectile flew over the town. A caretaker of a local school who said the force of the explosion shook classroom windows approximately 200 meters from the school gates.

Prime Minister Mateusz Morawiecki convened the Committee of the Council of Ministers for National Security and Defense Affairs to discuss the incident, a government spokesman said.

'Important that all facts are established'

With little publicly known about the origin of the projectiles, the incident has raised speculation that it could represent accidental spillover from Russia's war in Ukraine. Ukrainian President Volodymyr Zelensky has already blamed Russia, describing the fatal explosion as a "significant escalation" -- a claim that Polish authorities did not confirm.

Russia's Defense Ministry has denied targeting the border, and called the reports by Polish media "a deliberate provocation in order to escalate the situation," according to a short statement late Tuesday.

"The statements of the Polish media and officials about the alleged fall of 'Russian' missiles in the area of ??the settlement of Przewodow is a deliberate provocation in order to escalate the situation," it said, adding that "there were no strikes made on targets near the Ukrainian-Polish state border."

It added that the photos of wreckage published by the Polish media "from the scene in the village of Przewodow have nothing to do with Russian weapons."

Kremlin spokesperson Dmitry Peskov meanwhile told CNN he had no information on an explosion in Poland.

NATO Secretary General Jens Stoltenberg said Tuesday that it was "important that all facts are established," after speaking with Polish President Andrzej Duda about the explosion.

"I offered my condolences for the loss of life. NATO is monitoring the situation and Allies are closely consulting. Important that all facts are established," Stoltenberg said in a post on Twitter.

NATO allies respond

NATO allies responded with concern to the reports. Some were circumspect in their statements, neither speculating or confirming the origin of the projectile.

US President Joe Biden, the White House says, "has been briefed on the reports out of Poland and will be speaking with President Andrzej Duda of Poland shortly."

A senior White House official says they do not have confirmation of any rocket or missile strike in Poland, but that US officials are currently working to try and figure out exactly what has happened.

State Department principal deputy spokesperson Vedant Patel echoed that the US cannot confirm the reports of missiles hitting Polish territory and killing two.

"We have seen these reports out of Poland and are working with the Polish government and our NATO partners to gather more information," Patel said at a press briefing. "We can't confirm the reports or any of the details at this time"

French President Emmanuel Macron has called for talks at the G20 summit on Wednesday, which Biden is attending, following reports of the incident in Poland, according to an Elysee Palace spokesman on Tuesday.

A French defense source told CNN that France was being "extremely cautious" and that officials would not comment until they were able to "analyze all the available information."

A UK Foreign Office spokesperson said they were "investigating these reports and liaising closely with Allies."

Baltic NATO states were more strident in their statements, stressing readiness to defend NATO territory.

Estonia called the news "most concerning," according to a Twitter post from the Estonian Ministry of Foreign Affairs.

"Estonia is ready to defend every inch of NATO territory," it added.

Lithuanian President Gitanas Nauseda has said he was concerned by the news, and that "Lithuania stands in strong solidarity with Poland."

"Every inch of NATO territory must be defended!" he added on social media.

Latvian Defense Minister Artis Pabriks afforded blame on Russia, even though there has been no confirmation from Polish authorities that Russian missiles landed on Polish territory.

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

No discussion of 'hypotheticals'

The North Atlantic Treaty Organization is a group of 30 North American and European nations. According to NATO, its purpose "is to guarantee the freedom and security of its members through political and military means."

The alliance was created in 1949 in response to the start of the Cold War. Its original purpose was to protect the West from the threat posed by the Soviet Union. Since the end of the Cold War, many former Soviet nations have joined NATO, much to the annoyance of Putin.

Article 4, which Poland is considering, is a consultation method that allows members to bring an issue, usually a security issue, affecting them for discussion at the North Atlantic Council, the alliance's decision making body.

"The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened," the article states.

The best-known aspect of the alliance is Article 5 of the treaty, which, if invoked, means "an attack against one Ally is considered as an attack against all Allies." It has only ever been invoked once, in response to the September 11, 2001 terrorist attacks on the United States.

However, the alliance can take collective defense measures without invoking Article 5 -- and has done this in the light of the Russian attack on Ukraine.

Russian President Vladimir Putin has long complained that NATO has, over time, expanded its borders by admitting Eastern European countries that were once part of the Soviet Union -- meaning Russia now shares a land border with the world's largest military alliance, thus reducing his geopolitical power in what was once Moscow's sphere of influence.

As recently as February, he was demanding that NATO scaled back to the borders of 1997, before the Baltic nations of Latvia, Lithuania and Estonia, the latter two of which border Russia, joined the alliance.

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