Around 200,000 people are currently trapped in Mariupol, encircled and under fire from relentless shelling since Russia launched its invasion of Ukraine on February 24.

"What now I see in Mariupol it's not about war, it's about genocide," Venediktova told AFP.

"Theatres of war have some rules, some principles. What we see in Mariupol, [are] no rules at all," she added.

Ukraine will work to officially "qualify" the siege as genocide on the international stage, she pledged.

Mariupol -- home to 450,00 before the war began -- has witnessed some of the most vicious violence nationwide.

Artillery has hammered a maternity hospital, flattened residential areas, and struck a theatre despite signs declaring children were sheltering inside.

Incoming humanitarian aid and outgoing evacuation convoys have struggled to penetrate the boundaries of the brutal siege.

"Mariupol residents have described a freezing hellscape riddled with dead bodies and destroyed buildings," a Human Rights Watch official said Monday.

Venediktova said Mariupol is now "a city hostage".

"It's impossible to give them food, water, electricity and [the] Russian Federation exactly knows what they are doing here," she added.

Mariupol is a key asset for Russia which would complete a "land bridge" between the Crimean peninsula annexed in 2014 and the Donbas region occupied by pro-Russia forces.

Seizing the port would also deprive Ukraine of a resupply route with the outside world on its southern Black Sea coast.