Talks between Ukraine and Russia are confrontational but moving forward, President Volodymyr Zelenskiy says, as the West plans to announce more sanctions against the Kremlin amid a worsening humanitarian crisis.

Intense Russian air strikes are turning besieged Mariupol into the “ashes of a dead land”, the city council said on Tuesday, as street fighting and bombardments raged in the port city.

Hundreds of thousands are believed to be trapped inside buildings, with no access to food, water, power or heat. Both civilians and Ukrainian troops were coming under Russian fire, said regional governor Pavlo Kyrylenko.

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.

But in an early morning address on Wednesday, Zelenskiy held out hope for negotiations, which have yielded little since the February 24 invasion began.

“It’s very difficult, sometimes confrontational,” he said.

“But step by step we are moving forward.”

Russian President Vladimir Putin’s incursion into Ukraine has forced more than 3.5 million people to flee, brought the unprecedented isolation of Russia’s economy, and raised fears of wider conflict in the West unthought-of for decades.

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

The port city 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.

Western nations plan to heap more pressure on the Kremlin.

Alongside European leaders, US President Joe Biden is expected to announce new sanctions against Russia and new measures to tighten existing ones when he visits Brussels this week.

The United States is preparing sanctions on more than 300 members of Russia’s lower house of parliament as soon as Thursday, according to The Wall Street Journal, which cited unnamed officials and internal documents.

Biden’s Europe trip is also set to include an announcement on joint action to enhance energy security on the continent, which is highly reliant on Russian gas, and a visit to Poland to show solidarity with Ukraine’s neighbour.

The US and its Western allies are also assessing whether Russia should remain within the Group of Twenty (G20) major economies, sources told Reuters.

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.

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.

Russia’s combat power in Ukraine has declined below 90 per cent of its pre-invasion levels, a senior US defence official said on Tuesday, without providing evidence. If confirmed, it would suggest heavy losses of weaponry and growing casualties.

The United Nations human rights office in Geneva on Tuesday said it had recorded 953 civilian deaths and 1557 injured since the invasion. The Kremlin denies targeting civilians.