The besieged Ukrainian city of Mariupol yielded up more horrors after six weeks of pummeling by Russian troops, with the mayor saying more than 10,000 civilians have died in the strategic southern port, their corpses “carpeted through the streets.”

As Russia pounded targets around Ukraine and prepared for a major assault in the east, the country’s leader warned President Vladimir Putin’s forces could resort to chemical weapons, and Western officials said they were investigating an unconfirmed claim by a Ukrainian regiment that a poisonous substance was dropped in Mariupol.

The city has seen some of the heaviest attacks and civilian suffering in the war, but the land, sea and air assaults by Russian forces fighting to capture it have increasingly limited information about what’s happening inside the city.