Russian forces have apparently taken hundreds of patients and staff hostage after capturing a hospital in the besieged Ukrainian city of Mariupol.

Donetsk regional governor Pavlo Kyrylenko said more than 400 people, including patients and doctors, were being held hostage in the basement of the intensive care hospital.

“Russians drove 400 people from neighbouring houses to the hospital and they can’t leave,” he said on the messaging app Telegram.

Mr Kyrylenko said the troops were using those inside the hospital as human shields.

The Ukrainian army’s General Staff said Russian troops were trying to block the city from its western and eastern outskirts.

“There are significant losses,” it said in a Facebook post.

Wednesday’s development came after an estimated 20,000 civilians finally managed to flee Mariupol along a humanitarian corridor. It is believed to be the biggest evacuation yet from the hard-hit city.

Mr Kyrylenko said the hospital was bombed but staff continued to treat patients in the basement as high-rise buildings burned around them.

The attack follows a deadly raid on a maternity and children’s hospital in the city last week.

There was also extended bombing of the Kyiv on Tuesday night. They followed Mayor Vitali Klitschko’s imposition of a 35-hour curfew, which runs until 7am on Thursday (4pm Thursday AEDT).

Air sirens were also reported in the western Ukraine city of Lviv on Tuesday night.

Also on Tuesday, Kharkiv Mayor Ihor Terekhov said more than 600 buildings had been destroyed in his city, Ukraine’s second largest, since Russia’s invasion began.

“Schools, nurseries, hospitals, clinics have been destroyed. The Russian army is constantly shelling [us] from the ground and the air,” he said in a televised address.

Ukrainian officials have repeatedly accused Russia of war crimes by going after civilians in residential buildings, schools and hospitals.

Despite the grim updates, Moscow has failed to capture any of Ukraine’s 10 biggest cities following its incursion that began on February 24.

Ukrainian officials have raised hopes the war could end sooner than expected, possibly by May, saying Moscow may be coming to terms with its failure to impose a new government by force and running out of fresh troops.

Ukrainian President Volodymyr Zelensky said on Wednesday (Australian time) that peace talks with Russia were beginning to “sound more realistic”.

“But time is still needed for the decisions to be in the interests of Ukraine,” he said in a video address ahead of the next round of talks.

Mr Zelensky has also conceded Ukraine is unlikely ever to join NATO – the country’s potential membership of the key Western security alliance was one of Russian President Vladimir Putin’s main issues of contention before his invasion.

Mr Zelensky told Britain’s Joint Expeditionary Force overnight that Ukraine was still seeking security guarantees short of joining NATO.

“For years we heard about the apparently open door, but have already also heard that we will not enter there, and these are truths and must be acknowledged,” he said.

Talks between Ukrainian and Russian negotiators continued on Wednesday, aimed at a potential ceasefire as the Russian onslaught continues.

Ukrainian negotiator Mykhailo Podolyak said there were “fundamental contradictions” during the talks, but “certainly room for compromise”.

Kremlin spokesman Dmitry Peskov said it was too early to predict progress in the talks.

“The work is difficult, and in the current situation the very fact that [the talks] are continuing is probably positive.”

Mr Zelensky will address the US Congress later on Wednesday.

US Senate labels Putin a ‘war criminal’

Ahead of that, the US Senate has unanimously passed a resolution condemning Russian President Vladimir Putin as a war criminal, a rare show of unity in the deeply divided Congress.

Tuesday’s resolution, introduced by Republican Senator Lindsey Graham and backed by senators of both parties, encouraged the International Criminal Court in The Hague and other nations to target the Russian military in any investigation of war crimes committed during Russia’s invasion of Ukraine.

“All of us in this chamber joined together, with Democrats and Republicans, to say that Vladimir Putin cannot escape accountability for the atrocities committed against the Ukrainian people,” Democratic Senate Majority Leader Chuck Schumer said in a speech on the Senate floor ahead of the vote.

Russia calls its actions a “special military operation” to demilitarise and “denazify” Ukraine.

Mr Putin has also called the country a US colony with a puppet regime and no tradition of independent statehood.

Earlier on Tuesday, Russia had put US President Joe Biden, Secretary of State Antony Blinken and other top officials on a “stop list” that bars them from entering the country.

Their names, together with US Defence Secretary Lloyd Austin, CIA chief William Burns, National Security Adviser Jake Sullivan and others, were included on a list of 13 individuals banned from Russia in response to sanctions imposed by the US on Russian officials.

Former secretary of state Hillary Clinton was also named, as was Biden’s son Hunter.

-with AAP