Moscow will formally annex four Russia-occupied regions of Ukraine at a Kremlin ceremony on Friday, President Vladimir Putin's spokesman said Thursday.

"Tomorrow in the Georgian Hall of the Grand Kremlin Palace at 15:00 (1200 GMT) a signing ceremony will take place on the incorporation of the new territories into Russia," spokesman Dmitry Peskov said.

He added that the Russian leader will make a major speech at the event.

Ukraine's Lugansk, Donetsk, Kherson and Zaporizhzhia regions are occupied by the Russian army, which Putin sent over the border in February.

Moscow organised what it called referendums in the four regions that it controls, with Kremlin-installed officials saying this week residents backed joining Russia.

All four Moscow-backed leaders of the regions said they were in Moscow and expecting a meeting with Putin.

The move comes eight years after Moscow annexed the Crimea peninsula from Ukraine and would mark a significant escalation in the conflict.

The West has warned Russia not to press ahead with he annexations, with the G7 saying it would "never recognise" the move.

Kyiv has asked for more military aid as a response.