Mariupol, encircled since the early days of Russia's five-week-old invasion, has been Moscow's main target in Ukraine's southeastern region of Donbas. Tens of thousands there are trapped with scant access to food and water.
The International Committee of the Red Cross (ICRC) sent a team on Friday to lead a convoy of about 54 Ukrainian buses and other private vehicles out of the city but they turned back, saying conditions made it impossible to proceed.
"They will try again on Saturday to facilitate the safe passage of civilians," the ICRC said. A previous Red Cross evacuation attempt in early March failed because the route was found to be unsafe.
Russia and Ukraine have agreed to humanitarian corridors during the war that have facilitated the evacuation of thousands of civilians.
The ICRC says its Mariupol operation has been approved by both sides, but major details are still being worked out.
In an early-morning video address, Ukrainian President Volodymyr Zelenskiy warned that Russian troops had moved toward the Donbas region and northeast in the direction of Kharkiv, Ukraine's second-largest city, where previous Russian strikes badly damaged urban areas.
"I hope they may still be solutions for the situation in Mariupol," Zelenskiy said. "The whole world has to react to this humanitarian catastrophe."
Russian President Vladimir Putin sent troops on February 24 for what he calls a "special operation" to demilitarise Ukraine.
The West calls it an unprovoked war of aggression that has killed thousands, uprooted a quarter of Ukraine's population and brought tensions between Russia and the United States to their worst point since the Cold War.
But the United States and its European allies have sent Ukraine military assistance, including an additional $US300 million ($A400 million) in aid announced by the Pentagon late on Friday. The new aid includes laser-guided rocket and anti-drone systems.
At peace talks this week, Russia said Donbas, where it has backed separatists since 2014, would now be the focus of its war efforts. Russian troops left behind shattered villages and their own abandoned tanks as they moved away from the capital Kyiv.
"You see that (the) enemy overestimates its potential around Kyiv at least. And we keep going forward liberating our cities and evacuating our people," said Deputy Interior Minister Yevhen Yenin.
After failing to capture a single major city, Russia has painted its draw-down of forces near Kyiv as a goodwill gesture in the peace negotiations.
Ukraine and its allies say Russian forces have been forced to regroup after suffering heavy losses due to determined Ukrainian resistance.
Across the border from Kharkiv in the Russian city of Belgorod, Moscow said Ukrainian helicopters struck a fuel depot on Friday, causing a huge fire. Ukraine denied responsibility for the incident, the first of its kind in the war.
The fire destroyed several oil tanks and would likely add short-term strain to Russia's already stretched logistics chains, particularly operations in Kharkiv, Britain's defence ministry said.
Kremlin spokesman Dmitry Peskov said the incident could jeopardise the peace talks. Russia would strengthen its western borders so it would not "cross anyone's mind to attack", Peskov said later.
As Ukrainian forces recaptured more territory around Kyiv on Friday, officials in the Black Sea port of Odesa said anti-air defences thwarted an attempted attack on critical infrastructure.Â
Odesa's governor, Maksym Marchenko, said three missiles had hit a residential district, causing casualties. He said the missiles were fired from an Iskander missile system in Crimea, the southern Ukrainian peninsula annexed by Russia in 2014. Russia denies targeting civilians.
Facing unprecedented sanctions over its invasion of Ukraine, Russia had threatened to cut off gas supplies to Europe unless buyers paid with roubles. Europe vowed to stay united against Russia's demand, and Moscow said it would not halt supplies until new payments were due later in April.
