HONG KONG/LONDON (NYTIMES) - The International Committee of the Red Cross said on Sunday (April 24) that it was "deeply alarmed" by the situation in Mariupol, calling for unimpeded access to help residents, including hundreds of wounded.

Russian forces continued on Sunday to drop heavy bombs from the air and direct artillery fire at the sprawling Azovstal steel factory, where a few thousand Ukrainian soldiers and civilians are holed up, according to Mykhailo Podolyak, an adviser to the Ukrainian president.

There were signs, he said, that Russian troops were gathering around the plant for a possible assault despite President Vladimir Putin's televised announcement last week in which he ordered his defence minister not to storm the facility but to blockade it instead.

"New facts about the crimes of the occupiers against our Mariupol residents are being revealed," President Volodymyr Zelensky of Ukraine said on Sunday. "New graves of people killed by the occupiers are being found. We are talking about tens of thousands of dead Mariupol residents."

At a news conference on Saturday, Zelensky said that if the civilians and soldiers in the steel factory were killed, Ukraine would "withdraw from any negotiation process".

The Red Cross has tried repeatedly in recent weeks to send a humanitarian convoy to Mariupol to help treat injured people and evacuate the remaining civilians, but violence has stymied the efforts. Tens of thousands of civilians are believed still to be in the city, out of a pre-war population of nearly half a million.

"Immediate and unimpeded humanitarian access is urgently required to allow for the voluntary safe passage of thousands of civilians and hundreds of wounded out of the city, including from the Azovstal plant area," the group said.

Ukrainian officials say that 20,000 civilians have been killed in the Russian assault on the southeastern port city, which Moscow's forces have failed to control completely despite two months of attacks.