The UN General Assembly meets Monday hours after Russia launched a deadly barrage of missile strikes at cities across Ukraine, as Western powers condemned Moscow's latest escalation of the conflict and seek to underscore its isolation.

The United Nations called the urgent meeting to discuss Russia's declared annexation of four partly-occupied Ukrainian regions, but the debate was set to be overshadowed by the attacks on Kyiv and other cities in one of the most punishing assaults on Ukraine in months.

Russian President Vladimir Putin vowed even more "severe" retaliation after the recent explosion that damaged a key bridge in Moscow-annexed Crimea -- an attack the Kremlin has blamed on Kyiv.

With Putin's actions in the spotlight, UN debate will open on a draft resolution denouncing what has been widely seen in the West as Russia's illegal seizure of Ukrainian land.

The decision to bring the annexation matter before the General Assembly, where the 193 UN members have one vote each -- and no one wields veto power -- was taken after Russia used its veto in a Security Council meeting September 30 to block a similar proposal.

The vote is expected no sooner than Wednesday.

"It's extremely important," said Olof Skoog, who, as EU ambassador to the world body, drafted the text in cooperation with Ukraine and other countries.

"Unless the UN system and the international community through the General Assembly react to this kind of illegal attempt, then we would be in a very, very bad place," the Swedish diplomat told reporters.

A failure by the General Assembly to act would give "carte blanche to other countries to do likewise or to give recognition to what Russia has done," he added.

A resolution draft seen by AFP condemns Russia's "attempted illegal annexations" of the Ukrainian regions of Donetsk, Luhansk, Zaporizhzhia and Kherson following "so-called referendums," and it stresses these actions have "no validity under international law."

It calls on all states, international organizations and agencies not to recognize the annexations, and demands the immediate withdrawal of Russian troops from Ukraine.

In response, Russia wrote to all member states in a letter attacking "Western delegations" whose actions "have nothing to do with protection of international law and the principles of the UN Charter."

"They only pursue their own geopolitical objectives," said the letter, signed by Russian ambassador Vassily Nebenzia.

Nebenzia said that given the circumstances, the General Assembly should vote by secret ballot -- a highly unusual procedure normally reserved for matters like electing the rotating members of the Security Council.

- 'A bit of desperation' -
 

"It doesn't suggest a high degree of confidence in the outcome if Russia is seeking to obscure the vote count," a senior official in US President Joe Biden's administration told reporters. "It does suggest a bit of desperation."

UN Secretary-General Antonio Guterres bluntly denounced the annexations.

"It stands against everything the international community is meant to stand for," he said.

"It flouts the purposes and principles of the United Nations. It is a dangerous escalation. It has no place in the modern world. It must not be accepted."

During last month's Security Council vote, no other country sided with Russia, though four delegations -- China, India, Brazil and Gabon -- abstained.

Some developing countries have complained the West is devoting all its attention to Ukraine, and others might be tempted to join them this week.

The vote will provide a clear picture of exactly how isolated Russia has become. Given the high stakes, backers of the draft are going all out to win over potential abstentionists.

"It's going to be tough," a senior European diplomat said, speaking on condition of anonymity.