Zaporizhzhia - Vladimir Putin proclaimed Russia's illegal annexation of a swathe of Ukraine in a pomp-filled Kremlin ceremony, promising Moscow would triumph in its "special military operation" against Kyiv even as some of his troops faced potential defeat.

The Russian president's proclamation of Russian rule over 15% of Ukraine - the biggest annexation in Europe since World War Two - has been firmly rejected by Western countries and even many of Russia's close allies.

The territory that Russia control amounts to more than 90,000 square km, equal to the size of Hungary or Portugal.

It comes as Russian forces in one of the four regions being annexed face being encircled by Ukrainian troops after Putin ordered a massive mobilisation drive to get hundreds of thousands of Russian men to the front.

In one of the toughest anti-American speeches he has delivered in more than two decades in power, Putin signalled he was ready to continue a battle for a "greater historical Russia" using whatever tools he had at his disposal and slammed the West as neo-colonial and Satanist.

"Truth is on our side. Russia is with us!" Putin told his country's political elite, who had gathered in one of the Kremlin's grandest halls to watch what him sign documents annexing four Ukrainian regions.

He said the United States had set a precedent when it had dropped two atomic bombs on Japan in 1945, but stopped short of issuing new nuclear warnings against Ukraine himself, something he has done more than once in recent weeks.

The ceremony culminated in the 69-year-old leader chanting "Russia Russia!" as he clasped the hands of the four Russian-backed officials he wants to run the annexed regions, which Ukraine is fighting to win back.

Putin said Russia and the four regions would defeat Ukraine together.

Zelenskiy and the West have condemned referendums that Moscow held in the four Ukrainian regions - and said showed big majorities to join Russia - as illegal shams, while several dozen Ukrainians interviewed by Reuters in the last week said that only people they described as "Russian collaborators" had voted, with most people boycotting them.

U.S. President Joe Biden condemned what he called Russia's "fraudulent attempt" to annex sovereign Ukrainian territory, which he said was a flagrant violation of international law and said new U.S. sanctions would hurt those who provided political or economic support to the annexation drive.
-Reuters