ITV News Global Security Editor Rohit Kachroo reports on the potential consequences of a reported Russian missile strike that killed two people inside Poland
The Polish government has said two people died on Tuesday, with local media reporting a projectile struck an area where grain was drying in Przewodów, a village near the border with Ukraine.
Russia’s Defence Ministry called reports that Russian missiles had hit Poland a "provocation" and said there were no strikes by Russian weapons near Ukraine-Poland border.
Kremlin spokesman Dmitry Peskov told ITV News he did not have "any information on that" when asked if the reported strike was an accident or deliberate.
However, Polish government spokesman Piotr Mueller confirmed top leaders were holding an emergency meeting due to a “crisis situation”, while officials said the country is increasing the readiness of its military.
The Polish government said in a statement that Foreign Minister Zbigniew Rau summoned the Russian ambassador and “demanded immediate detailed explanations.”
In his nightly address, President Volodymr Zelenskyy said the reported strikes in Poland were “a very significant escalation” that offered proof that “terror is not limited by our state borders”.
"We need to put the terrorist in its place. The longer Russia feels impunity, the more threats there will be for everyone within the reach of Russian missiles,” Mr Zelenskyy said.
US President Joe Biden spoke to his Polish counterpart by phone on Tuesday evening from Bali where world leaders are currently gathered for the G20 summit in Bali.
He offered "full US support for and assistance with Poland's investigation" according to a White House readout on the president's call with Andrzej Duda.
Prime Minister Rishi Sunak said the UK was "urgently looking into reports of a missile strike in Poland" and said the country would "support our allies as they establish what has happened".
President Volodymyr Zelenskyy called the apparent Russian missiles striking a site in Poland "a very significant escalation" of the war.
Among the theories being investigated is that one or more of the missiles were brought down by an air defence system, and landed just across the Polish border.
ITV News' Global Security Editor Rohit Kachroo said Russia striking Poland, a NATO member, is potentially "very serious".
"Under Article 5 of the Washington Treaty there is a collective defence - an attack against one ally is an attack against all. Therefore we’re all watching very closely to see how NATO responds," he said.
"We don’t know whether this was a deliberate attack, however, this will be potentially the biggest test of Article 5 in the history of NATO."
ITV News Europe Editor James Mates in Bali where world leaders are gathered for the G20 summit on what might be the global reaction to the attacks
Russia had unleashed one of its biggest bombardments yet across Ukraine - 85 missiles according to President Zelenskyy.
Residential buildings in Kyiv were targeted by a wave of missile, while authorities announced emergency blackouts after attacks on energy and other facilities knocked out power.
The Pentagon said it was aware of the reports but did not have any information to corroborate the reports.
In a press briefing on Tuesday, Brig. Gen. Pat Ryder, said: "We're not going to get ahead of ourselves here. We're going to get the facts. And when we have more to provide, we will."
He added: "But we have made it crystal clear that we will protect every inch of NATO territory."
In response to the reports, Russia top propagandist and RT boss Margarita Simonyan wrote on Twitter: "Poland has now got its own Belgorod region. How do you like it?"
Belgorod is a Russian region which has seen explosions since start of the war in Ukraine.
In a follow up tweet, she wrote: "Good evening 1962", a reference to the Cuban missile crisis when the USSR and the US came to brink of nuclear war.
Countries in the region, such as Estonia, Lithuania and Latvia expressed support for Poland.
Earlier in the day, Mr Zelenskyy said Russia fired most of its missiles at "our energy infrastructure,” and shut down power in many cities.
“We’re working, will restore everything. We will survive everything,” the president vowed.
His energy minister said the attack was “the most massive” bombardment of power facilities in the nearly nine-month-old Russian invasion, striking both power generation and transmission systems.
Likewise, neighbouring Moldova reported massive power outages after the strikes knocked out a key power line that supplies the small nation, an official said.
The minister, Herman Haluschenko, described the missile strikes as “another attempt at terrorist revenge” after military and diplomatic setbacks for the Kremlin.
He accused Russia of “trying to cause maximum damage to our energy system on the eve of winter”.
The aerial assault, which resulted in at least one death in a residential building in the capital, Kyiv, followed days of euphoria in Ukraine sparked by one of its biggest military successes - the retaking last week of the southern city of Kherson.
What is Article 5?
NATO was founded in 1949 when the US, the UK, Portugal, Norway, Netherlands, Luxembourg, Belgium, Italy, France, Iceland, Denmark and Canada signed the North Atlantic Treaty in Washington DC.
The alliance is defensive in nature, it is not supposed to orchestrate offensive operations.Central to the treaty is Article 5, which enshrined the principle of collective defence that means an attack against one ally is considered an attack against all allies.
It was invoked for the first time in the wake of 9/11, which obliged all member states to come to the US's aid.
As a consequence, a large global coalition was formed that led to the invasion of Afghanistan in 2001.
Ukraine has several agreements with Nato and aspires to join the alliance one day, but is not a member and so is not protected by Article 5 which meant nations were not obliged to step in when Russia attacked.
Meanwhile, Article 4 allows nations to call for urgent consultations when “the territorial integrity, political independence or security of any of the parties is threatened".
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know