Pictures by border guard show Azovstal bunker

STORY: The photographs showed service members gathered in a dark room around a table, some playing chess and reading.

The Ukrainian military said there was no let-up on Sunday in Russia's bombardment of the steel works in the southern port, where a few hundred Ukrainian fighters are holding out weeks after the city fell into Russian hands.

Russian forces pummeled the port city for nearly two months, turning Mariupol into a wasteland as the war in Ukraine escalated.

Groups of Ukrainian fighters and civilians took to the Azovstal works - a vast Soviet-era plant founded under Josef Stalin and designed with a labyrinth of bunkers and tunnels to withstand attack.

Civilians have been evacuated from the bunkers but Ukrainian fighters remain holed up there.