Russian President Vladimir Putin signed treaties on Friday to annex four Ukrainian regions partly occupied by his forces, escalating his seven-month war and taking it into an unpredictable new phase.

"This is the will of millions of people," he said in a speech before hundreds of dignitaries in the St George's Hall of the Kremlin. "People living in Luhansk, Donetsk, Kherson region and Zaporizhzhia region are becoming our compatriots forever."

At a ceremony that Kyiv called a "Kremlin freak show" devoid of legal meaning, Putin delivered a 37-minute diatribe against the West, accusing it of "sheer Satanism," before signing the treaty documents with the Russian-backed heads of the four entities.

They then all clasped hands and chanted "Russia! Russia!" in unison with hundreds of dignitaries, who rose in a standing ovation.

The ceremony took place three days after the completion of hastily staged so-called referendums in which Moscow's proxies in the occupied regions claimed majorities of up to 99% in favor of joining Russia.

Ukraine and Western governments said the votes, announced only 10 days ago, had been conducted at gunpoint and were bogus and illegitimate.

Ukraine, the United States and the head of the United Nations had all said the annexation ceremony would have no legal value.

Putin urged Ukraine to cease military action and return to the negotiating table. Kyiv has vowed to recapture all the lands seized by Russia and said that Russia's decision to annex the territories has destroyed any prospect of talks.

The annexations mean that Russia, which had already seized Crimea from Ukraine in 2014, now lays claim to some 22% of Ukraine's territory, including parts that it does not control.

The Kremlin leader kept hundreds of assembled dignitaries waiting for 18 minutes before entering the imposing columned hall through a pair of golden doors opened by high-stepping guards, as a fanfare blared.

In his speech, he evoked the memory of Russian heroes from the 18th century to World War Two and repeated familiar accusations against the West, accusing it of colonial practices and recalling the use of nuclear weapons by the United States against Japan, which he called a "precedent," at the end of World War Two.

The hurried annexations mean that the front lines of the war will now run through territory that Russia is declaring as its own and that Putin has said he is ready to defend with nuclear weapons if necessary.

Some Western politicians called that a bluff – something Putin explicitly denied. The United States says it has warned Russia of catastrophic consequences if it does use a nuclear weapon.