EU leaders on Friday vowed they would "never recognise" Russia's illegal annexation of four more regions in Ukraine and accused the Kremlin of imperilling global security.

"We firmly reject and unequivocally condemn the illegal annexation by Russia of Ukraine's Donetsk, Luhansk, Zaporizhzhia and Kherson regions," the 27 leaders said.

Their statement was released as Russian President Vladimir Putin formally signed an accord that sees Moscow lay claim to the occupied territories.

"Russia is putting global security at risk," the EU leaders said, accusing Moscow of "wilfully undermining the rules-based international order and blatantly violating the fundamental rights of Ukraine to independence, sovereignty and territorial integrity, core principles as enshrined in the UN Charter and international law".

The leaders dismissed the "illegal" referendums staged by the Kremlin to justify its land grab and called on "all states and international organisations to unequivocally reject this illegal annexation".

The four territories create a crucial land corridor between Russia and the Crimean peninsula, which Moscow annexed in 2014.

"These decisions are null and void and cannot produce any legal effect whatsoever. Crimea, Kherson, Zaporizhzhia, Donetsk and Luhansk are Ukraine," the EU statement said.

The bloc said it remains committed to supporting Ukraine's "legitimate right" to regain control of all its territory within its internationally recognised borders.

Putin has warned he could use nuclear weapons to retain control of the territory.

"The nuclear threats made by the Kremlin, the military mobilisation and the strategy of seeking to falsely present Ukraine's territory as Russia's and purporting that the war may now be taking place on Russia's territory will not shake our resolve," the EU leaders said.

The EU has slapped repeated waves of unprecedented sanctions on Moscow over its war in Ukraine and has spent billions of euros on weapons for Kyiv.

The bloc is currently eyeing a new wave of economic measures against the Kremlin that would target Moscow's exports worth some $7 billion and seek to impose a price cap on Russian oil.

"We will strengthen our restrictive measures countering Russia's illegal actions. They will further increase pressure on Russia to end its war of aggression," the statement said.