Two people were killed after Russian missiles targeted at Ukraine crossed into NATO member state Poland, a senior U.S official has said.

Missile strikes on Tuesday left up to seven million homes without power across the besieged nation, with the Ukrainian presidential office reporting there were power cuts in the Kyiv, Vinnytsya, Volyn, Donetsk, Dnipropetrovsk, Zhytomyr, Kirovohrad, Lviv, Poltava, Rivne, Sumy, Ternopil, Kharkiv, Khmelnytskyy, Cherkasy, Chernivtsi and Chernihiv regions.

And it appears at least one missile struck inside neighbouring Poland, a member of the North Atlantic Treaty Organisation.

It was reported by Visegrad that two stray missiles hit a farm in Przewodów, with Polish PM Morawiecki & President Duda summoning a crisis meeting of the National Security Bureau.

A senior U.S official confirmed, on condition of anonymity, that missiles had crossed into Poland and that two people had been killed.

The Pentagon has said it cannot corroborate the report, Reuters news agency says. AFP reported that the body was "looking into" the situation.

A Polish government spokesman did not immediately confirm the information, but said leaders were meeting on "crisis situation."

NATO was formed during the Cold War and under its terms members agree to defend each other in the event of an attack by third party countries.

Ukraine is not a member of the alliance, though countries who form part of the bloc have been giving the country assistance in the form of weapons and money.

However it's feared that the deaths of two Polish citizens within the nation's territory could mark a serious escalation.

Article 5 of the NATO treaty states: "The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area."

A UK Foreign Office spokesman said: "We are investigating these reports and liaising closely with allies."