Ukrainian President Volodymyr Zelensky has called on Russian troops to surrender in an emotional video address.

“Please, listen to me,” Mr Zelensky urged Russian conscripts and officers in a speech posted to his Facebook on Tuesday (Australian time).

“You won’t be able to take anything from Ukraine. You will take lives – you are many – but yours will be taken too.

“Why should you die? What for?

“I know that you want to survive. We hear your conversations in the intercepts. We hear what you really think about this senseless war, about this disgrace and about your state.”

Mr Zelensky’s latest public address came as Russia’s advances in Ukraine appeared to have stalled – partly because of a much stronger than expected resistance from Ukrainian forces.

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A senior US defence official, who could not be named, said the Ukrainians had struck at Russian logistics and sustainment capabilities”, limiting the ability of the Kremlin to resupply its advancing troops.

Attacks on the cities of Chernihiv and Kharkiv remain stalled. But Chernihiv is now isolated, and there has been an increase in strikes from long-range fire missiles in Kharkiv.

In the south, Ukraine continues to defend the embattled city of Mariupol, though it remains isolated and under heavy bombardment.

Mr Zelensky said Russia had already lost 90 warplanes and that Russian troops “did not expect such resistance”.

“On behalf of the Ukrainian people, I give you a chance,” he said in a translation of his address shared by his office.

“If you surrender to our forces, we will treat you the way people are supposed to be treated – as people, decently.”

Mr Zelensky said some Russian troops were fleeing battlefields and abandoning equipment.

“Today, Russian troops are, in fact, one of the suppliers of equipment to our army,” he said.

US warning to China

The US has warned China against providing military or financial help to Moscow after its invasion of Ukraine, as sanctions on Russian political and business leaders mount and civilians seek to flee intense fighting on the ground.

Further talks between Ukrainian and Russian negotiators to ease the crisis were expected on Tuesday after discussions on Monday via video ended with no new progress announced.

Thousands have been killed in intense fighting and bombardments since Russian President Vladimir Putin ordered the invasion of Ukraine on February 24.

Russia calls its actions a “special military operation” to “denazify” the country and prevent genocide, a claim the US and its allies reject as a pretext for an unjustified and illegal attack.

According to US officials, Russia has asked for military and economic support from Beijing, which has signalled a willingness to provide aid.

Moscow denies that, saying it has sufficient resources to fulfil all of its aims. China’s foreign ministry has labelled the reports on assistance as “disinformation”.

“We have communicated very clearly to Beijing that we won’t stand by,” State Department spokesperson Ned Price said after US national security adviser Jake Sullivan met China’s top diplomat Yang Jiechi in Rome.

“We will not allow any country to compensate Russia for its losses.”

The seven-hour meeting was “intense” and reflected “the gravity of the moment”, according to a US official.

Mr Yang said Beijing regretted the war in Ukraine but would not accept attempts to “smear” China for its position, according to a statement from the Chinese side.

China also dismissed the US claims that it had expressed some openness to providing Russia with requested military and financial assistance as part of its war on Ukraine as US “disinformation.”

“China always calls for respecting the sovereignty and territorial integrity of all countries and abides by the United Nations Charter and principles,” Mr Yang told Mr Sullivan.

Beijing would continue to promote peace talks and had provided “emergency humanitarian aid” to Ukraine, he said.

Mr Yang also said it was also important to “straighten out the historical context of the Ukraine issue, get to the bottom of the problem’s origin, and respond to the legitimate concerns of all parties”.

Fears of chemical attack remain

Britain’s defence ministry says it believes Russia might still be planning to use chemical or biological weapons in Ukraine in response to a staged fake attack on Russian troops, without citing evidence.

US officials have made similar statements.

Russia has accused Ukraine of planning to use biological weapons. The United Nations on Friday said it had no evidence Kyiv had such a program.

On Monday, Moscow on Monday allowed the first convoy to escape besieged Mariupol, home to the worst humanitarian crisis of the conflict.

“In the first two hours, 160 cars left,” Andrei Rempel, a representative of the Mariupol city council told Reuters.

Local authorities say as many as 2500 civilians have died so far, a toll that cannot be independently confirmed.

The UN says more than 2.8 million people have left Ukraine since the start of the war.

Russia says it does not target civilians.

-with AAP