Attacks have resumed on Mariupol's steel works, despite hundreds of civilians remaining trapped inside, a Ukrainian officer has said, BBC reported.

The Azovstal plant was being shelled by "all kinds of weapons", National Guard commander Denys Shlega said on Monday.

On Sunday a number of civilians who had sheltered inside the last resistance stronghold managed to escape.

But "several dozen small children are still in the bunkers underneath the plant", the commander said.

The shelling on the plant in the southern port city, which has been under intense Russian bombardment for weeks, began as soon as the civilians who had been evacuated left, he told Ukrainian television, according to BBC.

On Monday evening, footage emerged apparently showing a massive fire at the Azovstal, in what social media users said was a result of Russian bombardment.

A first group of evacuees from the steelworks were expected to arrive in the Ukrainian-controlled city of Zaporizhzhia on Monday - but the rescue efforts have run into delays, the BBC's Laura Bicker, who is in the city, reports.

It is not clear what is causing the hold-up, she says. The convoy is on its way, but the buses have hundreds of miles to travel along a road which is - in part - rubble. They also have to go through a number of Russian checkpoints. 

Those who left Mariupol on Monday were evacuated with the support of the United Nations (UN) and the International Committee of the Red Cross, which organised an official convoy.

Russia said some evacuees had been taken to a village controlled by Moscow-backed separatists. But state media later reported that they would be free to travel onwards to Ukrainian-held territory if they wanted to.

Ukraine's President Volodymyr Zelensky welcomed the news that around 100 people were heading for Zaporizhzhia, which is about 140 miles (230km) north-west of Mariupol, BBC reported.

"Grateful to our team! Now they, together with UN, are working on the evacuation of other civilians from the plant," he wrote on Twitter.

Some people have spent many weeks sheltering in the Azovstal steelworks, with reports suggesting food, water and medicine supplies are all running low. 

"The situation has become a sign of a real humanitarian catastrophe," Ukraine's Deputy Prime Minister Iryna Vereshchuk said.

One Russian news report estimated the number of civilians still in the plant was more than 500, according to BBC.