ZAPORIZHZHIA (REUTERS) - As Dmytro Kartavov and his family joined thousands of people trying to flee the bombed-out city of Mariupol, one additional obstacle awaited as Russian troops sought to identify anyone fighting with Ukrainian forces defending the city.

"They stripped the men naked, looked for tattoos," said Kartayov, a 32 year-old builder, who said the troops paid particular attention to men's knees.

"I work, I do repairs, naturally my knees - these are working knees. They say - (you) climbed trenches, dug, and the like."

Speaking in a supermarket that has been turned into a reception centre in the Ukrainian-held town of Zaporizhzhia some 200km from Mariupol, he told Reuters the family left the besieged city to the west, reaching the port of Berdyansk by bus before crossing into Ukrainian-held territory on foot.

On the way, Kartavov said Russian soldiers checked men for signs they had been fighting with Ukrainian forces.

"They checked my forearms, looked at the hands, checked if I was shooting, whether or not there will be a callus here on the arms," he added.

Russia's defence ministry did not respond to a request for comment on the searches.

Mariupol, an industrial port city close to the breakaway Donetsk People's Republic, had a pre-war population of around 400,000, but the city has been devastated by regular bombardment since Russia invaded Ukraine on Feb 24.

Fierce resistance by Ukrainian forces including regular army troops and members of the Azov Battalion, a far-right militia now part of Ukraine's National Guard, have held off the assault for weeks, rejecting Kremlin demands to lay down their arms.

The fighting has killed around 5,000 civilians, according to city authorities, and forced thousands to flee either by private car or in organised convoys of buses into eastern Ukraine where pro-Russian separatists are in control, or to the north and west to territory controlled by the Ukrainians.

As the convoys try to leave, checks to identify fighters have become a regular part of the ordeal, witnesses said.

Vladimir Andreev, a 63-year-old pensioner from Mariupol and former employee of Metinvest steel company, travelled in a separate group that included his wife and their friend, Valentina Kirichek.

Andreev said the party was stopped around 17 times at various Russian checkpoints.

"At each checkpoint we were stopped ... We were checked, undressed. They checked our shoulders, arms ... (to see) if I had been taking part in the fighting."