(Adds Putin on mobilisation, call with Turkey's Erdogan)
Putin to sign annexation papers on Friday
May attend Red Square victory concert
Ukraine, West say annexation move illegal
U.S. EU sanctions to follow
Ukraine threatens Russian gains on battlefield
LONDON, Sept 29 (Reuters) - President Vladimir Putin will begin annexing four Ukrainian regions to Russia on Friday, a move the United Nations warned would mark a "dangerous escalation" and jeopardise prospects for peace, while Kyiv warned it would elicit a harsh response.
Moscow's planned annexation of 15% of Ukraine's territory, after what Kyiv and Western countries say were phoney referendums staged at gunpoint in Russian-occupied areas, shows Putin doubling down on his war despite suffering a major reversal on the battlefield this month and discontent at home over a widely criticised partial military mobilisation.
Putin said that "all mistakes" made in a call-up to reinforce Russia's military operation in Ukraine should be corrected,
his first public acknowledgment
that the "partial mobilisation" he announced last week had not gone smoothly.
He spoke late on Thursday, on the eve of a planned ceremony in the Kremlin and concert in Moscow's Red Square to mark the annexation, details of which sparked a fresh round of international condemnation.
"Any decision to proceed with the annexation of Donetsk, Luhansk, Kherson and Zaporizhzhia regions of Ukraine would have no legal value and deserves to be condemned," United Nations Secretary General Antonio Guterres told reporters.
U.S. Secretary of State Antony Blinken said Russia's votes were a sham and a "futile attempt to mask attempted land grab in Ukraine." Turkey's President Tayyip Erdogan pressed Putin in a call to take steps to reduce tensions in Ukraine.
Russia says the referendums were genuine and showed public support.
Ukrainian President Volodymr Zelenskiy, who will hold an emergency meeting with security and defence chiefs on Friday, promised a harsh response to a step he says has killed off chances of reviving peace talks.
The votes "are worthless and do not change reality. The territorial integrity of Ukraine will be restored," he said.
Ukraine's top general said he and the top U.S. Army commander in Europe had discussed the war and that he had received assurances of further support from Washington.
CEREMONY AND CONCERT
Putin's annexation ceremony will be held in one of the Kremlin's grandest halls with the pro-Russian figures Moscow considers to be leaders of the four Ukrainian regions.
Putin's spokesman Dmitry Peskov added Putin would deliver a major speech. He did not say whether Putin would attend the Red Square concert, as he did a similar event in 2014 after Russia proclaimed it had annexed Ukraine's Crimea region.
A tribune has already been set up on the Moscow square with giant video screens and billboards proclaiming "Donetsk, Luhansk, Zaporizhzhia, Kherson - Russia!"
A political adviser to the Ukrainian president said on Thursday the ceremony "does not make legal sense" and would be a "Kremlin freak show."
Washington and the European Union are set to impose additional sanctions on Russia over the annexation plan, and even some of Russia's close traditional allies, such as Serbia and Kazakhstan, say they will not recognise the move.
Putin has warned he could use nuclear weapons to defend Russian territory if necessary and on Thursday he said that the West stood ready to provoke "colour revolutions" and a "bloodbath" in any country, without naming a specific country.
With tens of thousands of Russian men fleeing abroad to escape Putin's military call-up, Finland shut one of the few remaining routes to Europe, saying it would no longer let Russians enter by land with EU tourist visas.
More than half of Russians felt fearful or anxious after hearing of the call-up, according to a poll released by the independent Levada Centre on Thursday.
The head of the upper house of the Russian parliament has said the chamber could consider the incorporation of the four regions on Oct. 4, three days before Putin's 70th birthday.
What Russia is billing as a celebration comes after Moscow has faced its worst setbacks of the war, with its forces routed in recent weeks in Ukraine's northeast.
Some military experts say Kyiv is poised to deliver another major defeat, gradually encircling the town of Lyman, Russia's main remaining bastion in the northern part of Donetsk province. Its fall could open the way for Ukrainian forces to launch attacks on swathes of territory that Russia now aims to annex.
"The situation looks increasingly precarious for Russian forces in Lyman as Ukrainian forces are about to cut them off," Carl Bildt, a former Swedish prime minister, said on Twitter.
"Another painful defeat for the Russian invasion forces is looming."
Kyiv has so far held back from disclosing details of the situation in Lyman. Russia's Defence Ministry said a day earlier that a Ukrainian offensive on Lyman had failed, with 70 Ukrainian soldiers killed.
Russian government officials have said that the four regions will fall under Moscow's nuclear umbrella once they have been formally incorporated into Russia.
The United States has unveiled a $1.1 billion weapons package for Ukraine that brings U.S. security aid to $16.2 billion.
Mystery continued to swirl over an apparent attack on the Nord Stream 1 and 2 pipelines under the Baltic Sea, built to carry Russian gas to Europe though already shut.
At least three explosions which sent hundreds of thousands of tonnes of methane gas jetting to the surface this week have left the pipes severely damaged, possibly permanently. Sweden's coast guard said it found a fourth leak.
Western countries have called the incidents sabotage while stopping short of openly ascribing blame. Russia, which has denied involvement, said the incidents looked like acts of state-sponsored terrorism and the United States stood to gain.
(Reporting by Reuters bureaux; Writing by Andrew Osborn and Alexandra Hudson; Editing by Peter Graff and Toby Chopra)