The US has warned NATO forces are ready to enter conflict and defend member countries against Russian attacks as the Ukraine crisis continues to escalate.
Secretary of state Antony Blinken said "we are ready" for conflict if it "comes to us".
NATO is the military alliance currently made up of 28 European countries, plus the US and Canada. Ukraine is not a member, but is seeking to join as Russia's attack on the country continues.
Blinken's warning to Vladimir Putin comes as Russia continued its bombardment of Ukraine with the shelling of a nuclear power station in Zaporizhzhia in the south-east of the country.
The targeting of the plant - which the UK said could “directly threaten the safety of all of Europe” - continued even as emergency services tried to put out the resulting fire.
Blinken said NATO is ready to defend its member allies and territory: "Ours is a defensive alliance. We seek no conflict. But if conflict comes to us we are ready for it and we will defend every inch of NATO territory.
"And overnight, we've also seen reports about the attack against a nuclear power plant. This just demonstrates the recklessness of this war and the importance of ending it - and the importance of Russia withdrawing all its troops and engaging in good faith in diplomatic efforts."
Ukraine president Volodymyr Zelenskyy has been calling for Ukraine to be admitted into NATO since before the Russian invasion began.
Watch: Russian shelling damages Ukrainian nuclear power plant
He had told the Munich Security Conference last month that a "clear and feasible" timetable for admission was needed. Since the war began last week, Zelenskyy also appealed for immediate membership of the EU.
However, NATO has been reluctant to allow Ukraine to join, with Russia making it clear this would be regarded as an act of provocation.
Article 5 of NATO's founding treaty states an "attack against one ally is considered as an attack against all allies". Therefore, it would lead to the prospect of all-out war with Russia.
NATO, explaining why it hasn't sent troops into Ukraine, has said: "NATO’s actions are defensive, designed not to provoke a conflict but to prevent a conflict. It is important to make sure that there is not an even bigger crisis in Europe where Russia challenges, or is threatening, or attacking any NATO country."
Meanwhile, General H.R. McMaster, the former US national security advisor, told BBC's Newsnight on Thursday that Ukraine has earned the right to be in NATO.
"We have to meet [Putin] with strength or else push until he meets strong resistance."