Encircled since the early days of Russia's five-week old invasion, Mariupol has been Moscow's main target in Ukraine's southeastern region of Donbas. Tens of thousands of people are trapped there 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 in a statement on Friday. A previous Red Cross evacuation attempt in early March failed.

An adviser to Ukrainian President Volodymyr Zelenskiy said he was hopeful about the Mariupol evacuations.

"I think that today or maybe tomorrow we will hear good news regarding the evacuation of the inhabitants of Mariupol," Oleksiy Arestovych told Ukrainian television.

Russia and Ukraine have agreed to humanitarian corridors during the war to facilitate the evacuation of civilians from cities, but have often traded blame when the corridors have not been successful.

Seven such corridors were planned for Saturday, Ukraine's Deputy Prime Minister Iryna Vereshchuk said, including one for people evacuating by private transport from Mariupol and by buses for Mariupol residents out of the city of Berdyansk.

After failing to take a major Ukrainian city since it launched the invasion on February 24, Russia says it has shifted its focus to the southeast, where it has backed separatists since 2014.

In an early morning video address, Zelenskiy said Russian troops had moved toward Donbas and the heavily bombarded northeastern city of Kharkiv.

"I hope there may still be solutions for the situation in Mariupol," Zelenskiy said.

In Chuhuiv, a city in Kharkiv province, two young women sat on neighbouring hospital beds, limbs bandaged and pinned in metal braces, survivors of an attack on a bus that they said was carrying around 20 civilians.

The war has killed thousands, uprooted a quarter of Ukraine's population and devastated cities such as Mariupol.

Russia denies targeting civilians in what President Vladimir Putin calls a "special military operation" aimed at demilitarising and "denazifying" Ukraine.

Ukraine calls it an unprovoked war of aggression and Western countries have imposed sweeping sanctions in an effort to squeeze Russia's economy.

British military intelligence on Saturday said Ukrainian forces continued to advance against withdrawing Russian forces near Kyiv, and that Russian troops had abandoned Hostomel airport in a northwest suburb of the capital, where there has been fighting since the first day.

The British daily assessment also said Ukrainian forces had secured a key route in eastern Kharkiv after heavy fighting.

Russia has depicted its drawdown of forces near Kyiv as a goodwill gesture in peace negotiations. Ukraine and its allies say Russian forces have been forced to regroup after suffering heavy losses.

In the early hours on Saturday, Russian missiles hit two cities - Poltava and Kremenchuk in central Ukraine, Dmitry Lunin, head of the Poltava region, wrote in an online post.

He said infrastructure and residential buildings were hit in the region east of Kyiv, but he had no casualty estimates. Reuters could not immediately verify the report.

In the Dnipro region in southwestern Ukraine, missiles hit an infrastructure facility, wounding two people and causing significant damage, Valentyn Reznichenko, head of the region, said in an online post.

Russia's defence ministry said high-precision air-launched missiles had disabled military airfields in Poltava and Dnipro.

Before dawn on Saturday, as sirens sounded across Ukraine, the Ukrainian military reported Russian air strikes on the cities of Sievierodonetsk and Rubizhne in Luhansk.

In that eastern region and neighbouring Donetsk, pro-Russian separatists declared breakaway republics that Moscow recognised just before its invasion.

The Ukrainian military also said defenders repulsed multiple attacks in Luhansk and Donetsk on Friday and that Russian units in Luhansk had lost 800 troops in the past week alone. Reuters was unable to verify those claims.

Australian Associated Press