LVIV/KYIV: Intense Russian air strikes are turning besieged Mariupol into the "ashes of a dead land", the city council said on Tuesday, as the United States and Europe planned more sanctions to punish Moscow for its invasion of Ukraine.

Street fighting and bombardments raged in Mariupol, Ukrainian officials said, a day after it rejected an ultimatum from Russia to surrender. Hundreds of thousands are believed to be trapped inside buildings, with no access to food, water, power or heat.

Russian forces and Russian-backed separatist units had taken about half of the port city, normally home to around 400,000 people, Russia's RIA news agency said, citing a separatist leader.

Street fighting was taking place in the city, and both civilians and Ukrainian troops were coming under Russian fire, said regional governor Pavlo Kyrylenko.

"There is nothing left there," Ukrainian President Volodymyr Zelenskiy said in a video address to Italy's parliament.

Mariupol Deputy Mayor Sergei Orlov told CNN the city was under a full blockade and had received no humanitarian aid.

"The city is under continuous bombing, from 50 bombs to 100 bombs Russian aircraft drops each day... A lot of death, a lot of crying, a lot of awful war crimes," Orlov said.

Mariupol has become the focus of the war that erupted on Feb. 24 when Russian President Vladimir Putin sent his troops over the border on what he calls a "special military operation" to demilitarise Ukraine and replace its pro-Western leadership.

It lies on the Sea of Azov and its capture would allow Russia to link areas in the east held by pro-Russian separatists with the Crimean peninsula, annexed by Moscow in 2014.

Putin's 27-day long incursion into Ukraine has forced more than 3.5 million to flee, brought the unprecedented isolation of Russia's economy, and raised fears of wider conflict in the West unthought-of for decades.

Western nations plan to heap more economic pressure on the Kremlin.

U.S. President Joe Biden will join allies in applying additional sanctions and tightening existing ones during his trip to Europe this week, national security adviser Jake Sullivan said on Tuesday.

The trip will also include an announcement on joint action to enhance energy security in Europe, which is highly reliant on Russian gas, and Biden will show solidarity with Ukraine's neighbour, Poland, with a visit to Warsaw.

Having failed to seize the capital Kyiv or any other major city with a swift offensive, Russia is waging a war of attrition that has reduced some urban areas to rubble and prompted Western concern that the conflict could escalate, even to a nuclear war.

Russia's security policy dictates that the country would only use such weapons if its very existence were threatened, Kremlin spokesman Dmitry Peskov told CNN on Tuesday.

"If it is an existential threat for our country, then it (the nuclear arsenal) can be used in accordance with our concept," he said.

Earlier he said "no one" had ever thought the operation in Ukraine would take just a couple of days and the campaign was going to plan, TASS news agency reported.


'HUMANITARIAN CATASTROPHE'

The United Nations human rights office in Geneva said on Tuesday it had recorded 953 civilian deaths and 1,557 injured since the invasion. The Kremlin denies targeting civilians.

Western officials said Russian forces were stalled around Kyiv but making some progress in the south and east. Ukrainian fighters are repelling Russian troops in some places but cannot roll them back, they said.

A Reuters team that reached a Russian-seized part of Mariupol on Sunday described a wasteland of charred apartment blocks and bodies wrapped in blankets lying by a road.

Ukraine says Russian shells, bombs and missiles have struck a theatre, an art school and other public buildings, burying hundreds of women and children sheltering in cellars.

Deputy Prime Minister Iryna Vereshchuk, speaking on Ukrainian television on Tuesday, demanded the opening of a humanitarian corridor for civilians. She said at least 100,000 people wanted to leave Mariupol but could not.

Referring to Russia's earlier demand that the city surrender by dawn on Monday, Vereshchuk said: "Our military are defending Mariupol heroically. We did not accept the ultimatum. They offered capitulation under a white flag."

Kyiv accused Moscow of deporting residents of Mariupol and separatist-held areas of Ukraine to Russia. This includes the "forcible transfer" of 2,389 children to Russia from the Luhansk and Donetsk regions, Prosecutor General Iryna Venediktova said.

Moscow denies forcing people to leave, saying it is taking in refugees.

In Kherson, a city under Russian control, Ukrainian officials said Moscow's forces were preventing supplies from reaching civilians.

"Kherson's 300k citizens face a humanitarian catastrophe owing to the Russian army's blockade," foreign ministry spokesperson Oleg Nikolenko said on Twitter.

Russia did not immediately comment on the situation in Kherson.

Zelenskiy warned the crisis in Ukraine, one of the world's biggest grain exporters, would bring famine elsewhere.

"How can we sow (crops) under the strikes of Russian artillery?" he told Italian lawmakers. 

In an address overnight, he also drew attention to the death of Boris Romanchenko, 96, who survived three Nazi concentration camps during World War Two but was killed when his apartment block in besieged Kharkiv was shelled last week.

In killing Romanchenko, "Putin managed to 'accomplish' what even Hitler couldn't," Ukraine's Defence Ministry said.