Russia is expected to annex four occupied regions of Ukraine at a lavish Kremlin ceremony on Friday that follows a threat by President Vladimir Putin to use nuclear weapons in their defence.

Kremlin spokesman Dmitry Peskov told reporters that the annexations would be formalised at the ceremony and Putin would deliver a "major" speech.

It comes after foreign leaders critical of Russia voiced their opposition to the plan, with US President Joe Biden saying the United States would "never, never, never" recognise Russian sovereignty over the territories.

The Kremlin's atomic threats have not deterred a sweeping Ukrainian counter-offensive, which has been pushing back Russian troops in the east.

Kyiv's forces are on the doorstep of the Donetsk region town of Lyman, which Moscow's forces pummelled for weeks before capturing it this summer.

Putin has blamed the war in Ukraine on the West and said simmering conflicts in the former Soviet Union were the result of its collapse.

The rhetoric built on his now famous phrase that the fall of the USSR was a tragedy, and he has recently suggested Moscow should again extend its influence over the former Soviet region.

The four regions' Kremlin-installed leaders who pleaded to Putin for annexation this week assembled in the Russian capital ahead of the ceremony.

Their almost simultaneous requests came after they claimed residents had unanimously backed the move in hastily organised referendums that were dismissed by Kyiv and the West as illegal, fraudulent and void.

- 20 percent of Ukraine -
Ukraine said the only appropriate response from the West was to hit Russia with more sanctions and to supply Ukrainian forces with more weapons to keep reclaiming territory.

UN Secretary-General Antonio Guterres condemned the annexation plans as "a dangerous escalation" that "has no place in the modern world."

The UN Security Council will vote Friday on a resolution condemning the referendums, according to France, the council's current president, but it has no chance of passing due to Moscow's veto power.

Ukraine's President Volodymyr Zelensky called an "urgent" meeting of his national security council for Friday, his spokesman said, after the Kremlin announced the timing of the annexation ceremony.

The four territories -- Kherson and Zaporizhzhia in the south and Donetsk and Lugansk in the east -- create a crucial land corridor between Russia and the Crimean peninsula, annexed by Moscow in 2014.

Together, all five make up around 20 percent of Ukraine, whose forces in recent weeks have been clawing back ground.

Ukrainian troops have been progressing in the eastern Kharkiv region and recapturing territory in Donetsk. Military observers say Kyiv's forces are close to capturing Lyman.

Moscow's forces are striking back along the entire front line and officials in Kyiv said Thursday that Russian bombardment had killed three people in the Dnipropetrovsk region, five in Donetsk and wounded seven in the Kharkiv region.

Along with threats to use nuclear weapons, Putin announced a mobilisation of hundreds of thousands of Russians to bolster Moscow's army in Ukraine, sparking demonstrations and an exodus of men abroad.

Putin on Thursday called for mistakes with the draft to be "corrected", as discontent grows over the often-chaotic conscription push.

US indicts Russian tycoon on sanctions violations

The United States indicted Russian tycoon Oleg Deripaska, his Russian partner and two others Thursday over a scheme to obtain US citizenship for his two children in violation of US sanctions.

The Justice Department said in the indictment that the aluminum billionaire and ally of Russian President Vladimir Putin arranged with the others to help his partner Ekaterina Olegovna Voronina come to the US twice to give birth.

Those acts violated US sanctions set in 2018 against Deripaska because of his close ties to the Russian government and its seizure two years before of the Crimea region from Ukraine.

The Justice Department said that Russia-based Natalia Mikhaylovna Bardakova and US-based Olga Shriki, a naturalized US citizen, helped Deripaska arrange for Voronina to come to the US by private jet on a tourism visa in 2020 and again in 2022 while pregnant to give birth.

The first effort was successful, but in June 2022 Voronina was refused entry.

The two also worked together to manage billionaire Deripaska's properties in the United States and to launder some $3 million from the sale of one of his businesses, a music studio, out of the country, according to the indictment.

And at Bardakova's direction, Shriki did other sanctions-violating work for Deripaska, such as sending flowers to Deripaska's personal contacts, including an unnamed television host and a former Canadian member of parliament.

And they did shopping for Deripaska, buying him iPhones and t-shirts from the American Eagle chain.

In addition to violating sanctions, Voronina and Bardakova are charged with making false statements to US officials relating to their connections to Deripaska.

The Justice Department said that Shriki was arrested Thursday, but the three others remain at large.

"Today’s indictment reflects the FBI's commitment to use all of the tools at our disposal to aggressively pursue those who attempt to evade the United States’ economic countermeasures against the Russian government," FBI Director Christopher Wray said in a statement.

The indictment came nearly one year after FBI agents raided properties owned by Deripaska and worth millions of dollars in New York and Washington.

Deripaska owns aluminum giant Rusal and has major shareholdings in EN+ Group and power company Eurosibenergo.

In 2018 the Treasury Department said Deripaska is being investigated for money laundering, extortion and racketeering, and is tied to Russian organized crime and murder.