Ukraine’s President Volodymyr Zelensky says diplomatic efforts are under way to save the remaining fighters holed up inside the Azovstal steelworks in the city of Mariupol, as more civilians were evacuated from the bombed-out plant.

The defenders have vowed not to surrender. Ukrainian officials fear Russian forces want to wipe them out by Monday in time for Moscow’s commemorations of the former Soviet Union’s victory over Nazi Germany in World War II.

Ukraine said 50 civilians were evacuated on Friday, accusing Russia of violating a truce intended to allow dozens more still trapped underground to depart after weeks under siege.

Mariupol has endured the most destructive bombardment of the 10-week-old war, and the sprawling Soviet-era Azovstal plant is the last part of the city – a strategic southern port on the Azov Sea – still in the hands of Ukrainian fighters.

Zelenskiy said in a late-night video address that Ukraine was working on a diplomatic effort to save defenders barricaded inside the steelworks. It was unclear how many Ukrainian fighters remained in the steelworks.

“Influential intermediaries are involved, influential states,” he said, but provided no further details.

Ukrainian President Volodymyr Zelensky will join G7 leaders to discuss Russia’s invasion on Sunday, amid fears Vladimir Putin may be poised to use a historic day for a dramatic statement about his next plans.

Sunday’s online meeting will come on the day much of Europe, including Ukraine, celebrate the end of World War II in the continent on May 8.

Victory Day show of strength?

Russia celebrates its own victory over Nazi Germany a day later, usually with a massive show of strength of troops and military hardware. Twenty-seven million Soviet citizens died in WWII, by far the greatest loss of any country.

This year, the event has a particular significance, with Russia waging months of war against its neighbour Ukraine and still devoid of any real form of military victory that it can celebrate.

Mr Putin, whose Victory Day address echoes across Red Square each year, often uses the day to send messages of intent.

“Even in a normal year it’s a huge show of Russia strength, of Putin’s control and everything he stands for,” Ammon Cheskin of Glasgow University told the BBC. “That’s just amplified this year.”

Claims that Mr Putin will declare an end to the brutal campaign have been denied, as have reports that he will announce a full declaration of war or a mobilisation of Russian men. Russia’s military would not “artificially adjust” its actions to any particular date, Foreign Minister Sergei Lavrov said.

Ukrainian presidential adviser Oleksiy Arestovych said Russia was trying to finish off forces inside the plant in order to seize it by Monday as a gift for President Vladimir Putin in time for the World War II Victory Day holiday.

After Russia annexed Crimea in 2014, Mr Putin marked Victory Day with a speech about defeating fascism, before flying to the Black Sea port of Sevastopol to celebrate his new victory in front of thousands of onlookers.

Speculation about what he will do this year came as Ukraine mounted another desperate bid to rescue scores of non-combatants trapped beneath a rain of Russian shells and missiles in the hellscape ruins of Mariupol’s besieged steel works.

Ukraine’s deputy prime minister Iryna Vereshchuk confirmed early on Saturday (Australian time) that a humanitarian convoy carrying 50 women, children, and elderly people had made it out of the Azovstal steel plant.

She described the evacuation as “extremely slow” and accused Russia of delaying efforts by violating a local ceasefire allowing the civilians to leave.

“Tomorrow morning we will continue the evacuation operation,” Ms Vereshchuk said.

Moscow’s defence ministry has separately confirmed that 50 civilians were evacuated on Friday and that the operation would continue.

Bloody fighting had thwarted earlier efforts to get the civilians to safety.

Mariupol, a strategic southern port on the Azov Sea, has endured the most destructive siege of the 10-week-old war. The sprawling Soviet-era Azovstal steel plant is the last part of the city still in the hands of holdout Ukrainian fighters.

United Nations-brokered evacuations of some of the hundreds of civilians who had taken shelter in the plant’s network of tunnels and bunkers began a week ago, but were halted in recent days by renewed fighting.

Aid workers in the Ukraine village of Bezymennoye with evacuees from the Avostal plant, among them four children. Photo: AAP

Mariupol blockade ‘inhuman’: Zelensky

Mr Zelensky said on Friday that Mariupol was one of the most complicated points of the war so far, war, accusing Russians of blockades and torture with starvation.

At a virtual event held by policy institute Chatham House to discuss the war, Mr Zelensky was asked about the situation in Mariupol.

He said all international organisations – including those from Ukraine – were  prohibited from entering the area to provide water, food and other supplies to civilians. The Russian military’s treatment in Mariupol was “inhuman” and a “beastly attitude”, he said, according to a report from CNN.

Mr Zelensky also emphasised the importance of Mariupol, saying that if Russia killed civilians that could be exchanged as prisoners of war, then Ukraine could not longer have diplomatic talks with the invaders.

Putin’s dubious pledge

Mr Putin said Russia was prepared to provide safe passage for the civilians but reiterated calls for Ukrainian forces inside to disarm.

He declared victory in Mariupol on April 21 and ordered his forces to seal off the plant.

The Kremlin denies Ukrainian allegations that Russian troops stormed the plant in recent days and said humanitarian corridors were in place.

Aerial footage of the plant, released on Thursday by Ukraine’s Azov Regiment, showed three explosions striking different parts of the vast complex, which was engulfed in heavy, dark smoke.

Reuters verified the footage location by matching buildings with satellite imagery, but was unable to determine when the video was filmed.

The stubborn Ukrainian defence of Azovstal has underlined Russia’s failure to take major cities in a war that has united Western powers in arming Kyiv and punishing Moscow with the most severe sanctions ever imposed on a major power.

Economic measures from Washington and European allies have hobbled Russia’s $US1.8 trillion ($A2.5 trillion) economy while billions of dollars worth of military aid has helped Ukraine frustrate the invasion.

UN Security Council to back Ukraine statement

The United Nations Security Council, including Russia, has agreed to express “deep concern regarding the maintenance of peace and security of Ukraine” in the body’s first statement since Russia invaded its neighbour 10 weeks ago, diplomats say.

Statements of the Security Council are agreed by consensus.

The brief text drafted by Norway and Mexico was due to be formally adopted at a meeting later on Friday (early Saturday, Australian time), diplomats said.

“The Security Council expresses deep concern regarding the maintenance of peace and security of Ukraine,” it reads.

“The Security Council recalls that all Member States have undertaken, under the Charter of the United Nations, the obligation to settle their international disputes by peaceful means.”

“The Security Council expresses strong support for the efforts of the Secretary-General in the search for a peaceful solution,” reads the statement, which also requests UN chief Antonio Guterres brief the council again “in due course”.

The statement was agreed despite a diplomatic tit-for-tat that has escalated since Russia launched on February 24 what it calls a “special military operation” and what Mr Guterres blasted as Russia’s “absurd war”.

Russia vetoed a draft Security Council resolution on February 25 that would have deplored its invasion.

China, the United Arab Emirates and India abstained from the vote.

-with AAP