Emergency service workers rescue the residents after a missile strike in Odessa, Ukraine. Reuters

Kyiv prepared on Saturday for its first wartime visit from two top US officials, as Ukraine accused Russia of killing eight people, including an infant, in a strike on the southern city of Odessa that all but buried hopes of a truce for Orthodox Easter.

In his daily video address on Saturday night, Zelensky said he was preparing for "tomorrow's important talks with American partners". The State Department declined to comment on the highly sensitive trip by two of President Joe Biden's top cabinet members.

Their visit comes as Russian forces show no sign of easing their attacks and after the Odessa strike.

"Among those killed was a three-month-old baby girl. How did she threaten Russia? It seems that killing children is just a new national idea of the Russian Federation," Zelensky said.

Rescuers carry a woman out of a damaged building in Odessa, Ukraine, after a reported missile strike. AFP

"All those bastards will answer for every death."

And Russia's defence ministry also said it had targeted a major depot stocking foreign weapons near Odessa, attacks that upended the relative calm the city has enjoyed since the beginning of the war.

The ministry also charged that Ukrainian special services in Odessa were preparing a "provocation with the use of toxic chemical substances" that could then be blamed on Russia.

Western powers have accused Russia in the past of making such accusations as a cover or diversion for attacks its own forces are planning.

The Sunday visit by Secretary of State Antony Blinken and Defence Secretary Lloyd Austin will come at a symbolic moment — on the day the war enters its third month — and with fierce battles continuing in the country's east.

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It also comes as the situation in the shattered port city of Mariupol remains bleak. The latest of many attempts to evacuate civilians failed on Saturday, and the situation facing an embattled unit of Ukrainian fighters sheltering in tunnels under a sprawling steel mill there appeared increasingly desperate.

Meanwhile, days after Moscow declared victory in the southern city, it said its forces did not need to take a giant steel plant as Russia resumed its assault on the last Ukrainian defenders holed up there in Mariupol.

Ukrainian President Volodymyr Zelensky said the country's army was not ready to try to break through the siege of the port city.

The trip by Secretary of State Antony Blinken and Defence Secretary Lloyd Austin, to discuss the types of weapons Ukraine needs, comes as the war enters its third month with thousands dead and millions displaced.

US Secretary of State Antony Blinken

"As soon as we have (more weapons), as soon as there are enough of them, believe me, we will immediately retake this or that territory, which is temporarily occupied," Zelenskiy told a Saturday evening news conference.

A series of European leaders have already travelled to Kyiv to meet President Volodymyr Zelensky and underscore their support, but the United States — a leading donor of finance and weaponry — has yet to send any top officials.

The White House has not confirmed any travel plans for Blinken and Austin. The State Department and Pentagon declined comment.

The attack on Mariupol, the biggest battle of the conflict, has raged for weeks. Capturing the city is seen as vital to Russia's attempts to link the eastern Donbas region with Crimea, the Black Sea peninsula Moscow seized in 2014.

Moscow-backed separatists have held territory in the Donbas region for years.

Firefighters work to extinguish a fire on a warehouse amid Russian bombardments in Kharkiv, Ukraine. AP

Ukraine estimates tens of thousands of civilians have been killed in Mariupol and says 100,000 civilians are still there. The United Nations and Red Cross say the civilian toll is at least in the thousands.

A new attempt to evacuate civilians failed on Saturday, an aide to Mariupol's mayor said.

Zelensky also accused Russia of being a terrorist state and of acting like Nazis in the shattered port city of Mariupol, which has been devastated by weeks of intense bombardment.

"New facts about the crimes of the occupiers against our Mariupol residents are being revealed. New graves of people killed by the occupiers are being found. We are talking about tens of thousands of dead Mariupol residents," he said.