Russian missiles target Ukraine city of Mykolaiv after more than 50 countries pledge more military aid

Russia has unleashed a barrage of missiles on the Ukrainian city of Mykolaiv, after the United Nations General Assembly condemned Moscow's attempted annexation of four Ukrainian areas and Kyiv's allies committed more military aid.

"A five-storey residential building was hit, the two upper floors were completely destroyed, the rest – under rubble. Rescuers are working on the site," Mayor Oleksandr Senkevich said in a social media post on Thursday, adding the southern city was "massively shelled".

A shipbuilding centre and a port on the Southern Bug river off the Black Sea, Mykolaiv has suffered heavy Russian bombardments throughout the war.

In New York, three-quarters of the 193-member General Assembly - 143 countries - voted on Wednesday in favour of a resolution that called Moscow's move illegal, deepening Russia's international isolation.

Only four countries joined Russia in voting against the resolution - Syria, Nicaragua, North Korea and Belarus. Thirty-five countries abstained from the vote, including Russia's strategic partner China, while the rest did not vote.

Ukrainian President Volodymyr Zelenskyy said on Twitter he was "grateful to 143 states that supported historic #UNGA resolution ... (Russia's) attempt at annexation is worthless."

In Brussels, more than 50 Western countries met to pledge more military aid to Ukraine, especially air defence weapons, on the heels of heavy retaliatory strikes this week ordered by Russian President Vladimir Putin in response to an explosion on a bridge in Crimea.

Pledges from allies included an announcement by France that it would deliver radar and air defence systems to Ukraine in the coming weeks. Britain pledged air defence missiles, and Canada said it would provide artillery rounds among other supplies.

Since Monday's attacks, Germany has sent the first of four IRIS-T SLM air defence systems, while Washington said it would speed up delivery of a promised NASAMS air defence system.

Mr Zelenskyy said the increased aid would strengthen the counteroffensive.

"The more assistance Ukraine gets now, the sooner we'll come to an end to the Russian war," Mr Zelenskyy said by video to a forum during International Monetary Fund and World Bank annual meetings in Washington.

Moscow in September proclaimed its annexation of four partially occupied regions in Ukraine - Donetsk, Luhansk, Kherson and Zaporizhzhia - after staging what it called referendums. Ukraine and Western nations have denounced the votes as illegal and coercive.

The General Assembly vote followed a veto by Russia last month of a similar resolution in the 15-member Security Council.

Russia's UN Ambassador Vassily Nebenzia told the General Assembly ahead of the vote that the resolution was "politicised and openly provocative," adding that it "could destroy any and all efforts in favour of a diplomatic solution to the crisis."

The moves at the United Nations mirror what happened in 2014 after Russia annexed Ukraine's Crimea. The General Assembly then adopted a resolution declaring the referendum invalid with 100 votes in favour, 11 against and 58 formal abstentions.

The United States and other Western countries lobbied ahead of Wednesday's vote. They won dozens more votes than compared with the 2014 result, and improved on the 141 countries who voted to denounce Russia and demand it withdraw its troops from Ukraine within a week of its invasion.

Air raid sirens sounded across parts of Ukraine for a third day on Wednesday. The Ukrainian governor of partially occupied Donetsk province said seven people were killed in Russian shelling of a market in the frontline town of Avdiivka. Reuters was not able to verify battlefield reports.

There were reports of some shelling elsewhere, but no sign of the countrywide strikes of the previous two days. Pope Francis denounced the bombings, part of what he called a "hurricane of violence".

A senior NATO official said a Russian nuclear strike would change the course of the conflict and almost certainly trigger a "physical response" from Ukraine's allies - "and potentially from NATO itself."

Mr Zelenskyy said Ukraine needed about US$55 billion ($88 billion) in financial support next year - US$38 billion ($61 billion) to close the budget deficit and $US17 billion ($A27 billion) to rebuild critical infrastructure such as schools and housing.

As his forces have lost ground on the battlefield since September, Mr Putin has escalated the conflict, ordering the call-up of hundreds of thousands of reservists, proclaiming the annexation of occupied Ukrainian territory and repeatedly threatening to use nuclear weapons to protect Russia.

US President Joe Biden said on Wednesday he doubted Putin would use a nuclear weapon.

Mr Putin is a "rational actor who has miscalculated significantly," Mr Biden said in a CNN interview, saying he believed the Russian president wrongly expected his invading troops to be welcomed.