Nato backed Ukraine to retake the territory formally claimed by Vladimir Putin on Friday night after the Russian president held a lavish ceremony inside the Kremlin to celebrate its annexation.
Mr Putin pledged to use “all means” to defend four new territories that he declared part of Russia during a speech and patriotic concert in Moscow.
The annexation brought the regions under Russia’s formal control, giving Moscow licence to use nuclear weapons to defend them.
Jens Stoltenberg, Nato’s Secretary-General, said that the military alliance’s support for Ukraine was “unwavering” and it would not be deterred by Moscow’s apparent threat.
He added: “We remain resolute in providing support to Ukraine as it continues to defend itself against Russia’s aggression…For as long as it takes.”
It came as Ukraine’s armed forces were on the brink of encircling hundreds of Russian troops in the key stronghold of Lyman, northern Donetsk.
Dymitry Peskov, a Kremlin spokesman, said the borders were still in flux, as he refused to define the current frontiers amid numerous Ukrainian gains.
In a 37-minute speech in the Kremlin’s grand St George’s Hall Mr Putin blamed the "satanic" West for the conflict.
He also said the US had set the “precedent” for using nuclear weapons with the bombings of Hiroshima and Nagasaki at the end of the Second World War.
Jake Sullivan, the US National Security adviser, said there was a "risk" that Mr Putin would consider using nuclear weapons, given all of the “loose talk and sabre-rattling” by the Russian president.
But he added that Washington saw no indications about the "imminent use of nuclear weapons".
Mr Sullivan that the Biden administration had directly communicated the decisive response it would take if Moscow went down "that dark road", adding the US had "substantial" military forces in Europe ready for "any contingency".
President open to peace talks
Mr Putin said he was willing to entertain peace talks to bring an end to the conflict, but warned that the four regions - Donetsk, Luhansk, Kherson and Zaporizhzhia - would not be on the table.
“People made their choice, an unambiguous choice,” he said, in reference to the hastily-organised referendums designed to cement Moscow’s control of the occupied areas.
“The people living in Luhansk and Donetsk, Kherson and Zaporizhzhia are becoming our citizens. Forever,” Mr Putin added.
Kyiv instantly rejected Russia’s offer of peace talks, insisting it would not come to the negotiating table while Mr Putin was still in charge of Russia.
In response to the annexations, Volodymyr Zelensky, Ukraine’s president, on Friday announced his country had formally applied for fast-track membership of Nato.
“Today, here in Kyiv, in the heart of our country, we are taking a decisive step for the security of the entire community of free peoples,” Mr Zelensky said in a video message filmed outside his office in the capital.
“We are taking our decisive step by signing Ukraine’s application for accelerated accession to Nato.”
Nato remains unlikely to allow Ukraine to join, but Mr Stoltenberg said every country should have the right to determine its own security arrangements.
He added: “A decision on membership has to be taken by all 30 allies.”
West condemns latest provocation
Within minutes of Mr Putin's annexations, Western governments moved to condemn Russia’s latest provocation.
Liz Truss said: “Vladimir Putin has, once again, acted in violation of international law with clear disregard for the lives of the Ukrainian people he claims to represent.
“The UK will never ignore the sovereign will of those people and we will never accept the regions of Donetsk, Luhansk, Kherson and Zaporizhzhia as anything other than Ukrainian territory.”
Joe Biden denounced Russia’s “fraudulent attempt today to annex sovereign Ukrainian territory”.
He said Moscow was “violating international law, trampling on the United Nations charter, and showing its contempt for peaceful nations everywhere”.
A US-led resolution tabled on Friday night and condemning Russia's proclaimed annexation of parts of Ukraine was rejected at the UN.
Russia's was the only vote against the resolution, while China, Gabon, India and Brazil abstained. Ten nations, including Britain, supported the resolution.
Mr Putin's speech in the Kremlin was followed by a concert in the Red Square, with government supporters bused in to watch pro-Kremlin musicians in a flag-waving, patriotic show of strength.
Admiral Sir Tony Radakin, the head of Britain’s Armed Forces, warned on Friday that any battle between Russia and the West could spill over into space.