Poland said that a Russian-made missile fell in the eastern part of the country, killing two people in a blast that marked the first time in the war with Ukraine that Russian weapons came down on a NATO country.

Ukrainian President Volodymr Zelenskyy decried the strike as “a very significant escalation” of the war.

The Polish government said in a statement that Foreign Minister Zbigniew Rau summoned the Russian ambassador and “demanded immediate detailed explanations.”

Polish government spokesman Piotr Muller said some military units were put on alert while officials sought details.

The Russian Defense Ministry denied being behind “any strikes on targets near the Ukrainian-Polish border” and said in a statement that photos of purported damage “have nothing to do” with Russian weapons.

The tragedy occurred as Russia bombarded Ukraine with more than 100 rockets yesterday in a fresh attack.

According to AP, two of those rockets instead landed in a farm near the town of Przewodow, north of Lviv, which is Ukraine’s most western city and likely the intended target – but so far, Russia has denied it was behind the explosion on Polish soil.

The news was initially announced by an unnamed, senior US intelligence official, who spoke on condition of anonymity due to the volatility of the unfolding situation.

Polish government spokesman Piotr Muller has since spoken to the media, and confirmed the nation was raising readiness for military combat units in response, and that Poland was also looking into whether it will activate Nato article four, a consultation process where individual members can bring a certain issue before the organisation for discussion.

“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,” article four states.

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However, Muller stopped short of claiming Russia was behind the incident, saying only that it was a “serious” situation and that “there was an explosion in eastern Poland”, which killed two people.

Nato chief Jens Stoltenberg, who has already spoken with Polish President Andrzej Duda, has also issued a cautious response.

“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,” he said on Twitter.

It comes as experts have come forward to claim it was too early to say who fired the missiles, given both Russia and Ukraine could have been to blame.

“Who fired the missile is unclear,” the BBC quoted J Andrés Gannon, a security expert at the US Council on Foreign Relations, as saying.

He added that the missile may have been from an S-300 system.

“We know Russia has been using the S-300 for ground attacks even though it’s an air defence system, but Ukraine also uses them for air defence against cruise missiles.”

Grain dryers were hit by the missiles earlier this morning Australian time, with emergency services immediately racing to the scene and leaving Polish Armed Forces on “high alert”, with military planes hastily assembled in response.

Polish government spokesman Piotr Müller earlier confirmed Prime Minister Mateusz Morawiecki had gathered the Committee of the Council of Ministers for National Security and Defence Affairs “as a matter of urgency” in the wake of the attack.

Another Polish government spokesperson also told CNN the top-level meeting had been called over the “emergency situation”, but declined to provide further details as an investigation gets under way. AP also reported a Polish government spokesman as saying the nation was in the midst of a “crisis situation”.

The situation has caused widespread panic and speculation regarding possible retaliation, given Poland is a member of Nato.

According to Nato’s article five provisions, an attack against one NATO member is considered an attack against all allies, and can trigger a co-ordinated response from all states.

It is also understood that nearby Moldova was also impacted by the barrage of Russian missiles, with huge power outages hitting the nation after a major power line was destroyed.

Vladimir Putin’s Ukraine invasion has been raging since February. Photo / AP

Meanwhile, Ukrainian President Volodymyr Zelenskyy has also spoken out in the aftermath of the Russian bombardment, revealing most rockets had targeted the country’s “energy infrastructure”.

“We’re working, will restore everything. We will survive everything,” he said.

So far, global reactions to the strike have been limited, although CNN reports that US Defence Department Press Secretary brigadier general Pat Ryder said the US was “aware of the press reports alleging that two Russian missiles have struck a location inside Poland near the Ukraine border”, while the Department insisted the US “will defend every inch of Nato territory”.

Nearby Estonia, which was formerly part of the Soviet Union and which has a strained relationship with Russia, has publicly expressed support for Poland.

“Latest news from Poland is most concerning. We are consulting closely with Poland and other Allies. Estonia is ready to defend every inch of NATO territory,” the Estonian Ministry of Foreign Affairs tweeted.

“We’re in full solidarity with our close ally Poland.”

Other Baltic nations have also shared similar sentiments, with Lithuanian President Gitanas Nauseda tweeting that “Lithuania stands in strong solidarity with Poland” and that “every inch of Nato territory must be defended!”

War now ‘an extreme possibility’

Alexander Vindman, a retired US Army lieutenant colonel who was the director for European Affairs for the United States National Security and who has spoken extensively about the Ukraine invasion, also weighed into the “terrible development”.

He said the strike against Poland revealed a great deal about the “trajectory of Russia’s war”. “For those that have heard me speak on it, Russian strikes on NATO were an inevitability, in a long war scenario,” he tweeted.

“This incident may be an accidental strike. However, it is just as likely that Russia is signalling the West that the war can spillover to NATO, thus the US should limit military support to Ukraine and coerce Ukraine to negotiate. This incident is important from another perspective.

“There is a logic to escalation. Analysts considering the possibilities of war, WMD use, etc. (still many months from this point) would mark this incident as key I & W (indication and warning) that the West is headed towards a military confrontation with Russia.

“Recommendation: help Ukraine achieve military victories & compel Russia to negotiate, soonest, before this conflict hits more milestones & locks NATO & Russia into war. War remains an extreme possibility, but we continue to march in that direction the longer the war continues.”

Russia denies attack

However, Russia’s Defence Ministry has so far denied that the rockets were Russian in origin.

“Information provided in the Polish media and by officials about the alleged fall of ‘Russian’ missiles in the area of Przewodów is a deliberate provocation aimed at escalating the situation,” the Russian Defence Ministry said on its Telegram channel.

“No strikes on targets near the Ukrainian-Polish state border were made by Russian means of destruction,” the defence ministry also said in a statement, adding that debris at the scene “has nothing to do with Russian weapons”.

- Alexis Carey, news.com.au