The Azovstal steelworks has become the last defensive stronghold in the southern city, with Ukrainian forces continuing to defend the site against Russian forces.

Ukraine has not said how many soldiers are stationed at the plant, but Russia has put the number at about 2,000.

Russian state media outlets reported that 25 civilians had been evacuated from the plant on Saturday, although the deputy commander of the Ukrainian forces holding out inside said the number was 20.

Russia's RIA Novosti news agency said 19 adults and six children were brought out but gave no further details.

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About 100,000 civilians are still in the city, while up to 1,000 are living beneath a Soviet-era steel plant, according to Ukrainian officials.

The successful departure comes as the United Nations continues its work to broker its own civilian evacuation from the area.

Key developments:
• The Foreign Office is investigating reports that a British national has been detained by Russia after a video emerged showing a man in camouflage clothes being questioned
• The family of a former US Marine say he was killed while fighting alongside Ukrainian forces, in what would be the war's first known death of an American in combat
• Hollywood star Angelina Jolie is seen in the Ukrainian city of Lviv visiting refugees
• Vladimir Putin has been forced to merge and redeploy depleted units in Ukraine, UK defence chiefs say

Video and images from inside the steelworks was shared by two Ukrainian women who said their husbands were among the fighters refusing to surrender there.

The footage showed unidentified men with stained bandages, while others had open wounds or amputated limbs.

The women also said skeleton medical staff were treating at least 600 wounded people and they identified their husbands as members of the Azov Regiment of Ukraine's National Guard.

They added some of the wounds were rotting with gangrene.

In the video the men said that they eat just once daily and share as little as 1.5 litres of water a day among four people, and that supplies inside the besieged facility are depleted.

The women claimed the video was taken in the past week in the warren of passageways beneath the plant.

One shirtless man appeared to be in pain as he described his wounds - two broken ribs, a punctured lung and a dislocated arm that "was hanging on the flesh".

He said: "I want to tell everyone who sees this: If you will not stop this here, in Ukraine, it will go further, to Europe."

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The Azov Regiment of the Ukrainian National Guard is a seasoned volunteer force that is considered one of Ukraine's most capable units.

On Saturday, Ukrainian forces were also fighting village by village to hold back a Russian advance through the country's east.

Moscow has turned its attention to the region, known as the Donbas, after failing to successfully take control of the capital city, Kyiv.

'Thousands more' Russian soldiers will be killed in 'coming weeks'

During his nightly address, Ukraine's President Volodymyr Zelenskyy warned that Russian troops are "gathering additional forces" to launch new attacks.

However, he claimed Russia's President Vladimir Putin has lost "more than 23,000 soldiers" so far and "thousands more will be killed and thousands wounded" in the coming weeks.

"Our defenders have already destroyed more than 1,000 Russian tanks, almost 200 Russian planes, almost 2,500 armoured fighting vehicles," he said.

"Of course, the occupiers still have equipment in stock. Yes, they still have missiles to strike our territory. But this war has already weakened Russia so much that they have to plan for even fewer (pieces of) military equipment to take part in a parade in Moscow."

Mr Zelenskyy also spoke of missile strikes in the Dnipropetrovsk region and Odesa.

He referred to a rocket attack that destroyed an airport runway in Odesa, which the Ukrainian army said had been carried out by Russian forces.

PM is 'more committed than ever to ensuring Putin fails'

Meanwhile, Prime Minister Boris Johnson has promised to provide additional military aid to Ukraine after speaking to Mr Zelenskkyy on the phone.

The two leaders discussed the UK's "continued economic and humanitarian support" for the country and Mr Johnson is said to have told the president that he is "more committed than ever to reinforcing Ukraine and ensuring Putin fails".

A Downing Street spokesperson said they also discussed the situation in Mariupol and the progress of the UN-led effort to evacuate civilians.

Britain is one of the largest suppliers in Europe of arms to Ukraine, having already sent more than 5,000 anti-tank missiles, 1,360 anti-structure munitions, five air defence systems with more than 100 missiles, and 4.5 tonnes of plastic explosives, according to the Ministry of Defence.