Corpses are now “carpeted through the streets” of Mariupol after Russian troops killed more than 10,000 civilians over the past six weeks in their unsuccessful fight to capture the strategic southern port, the mayor said, while Western powers warned Tuesday of an ongoing buildup for a suspected Russian assault in Ukraine’s east.

The British Ministry of Defense said Russian forces are continuing to pull out of Belarus to support operations in eastern Ukraine, focused on the Donbas region, where Russian-allied separatists have claimed independence.

“Fighting in eastern Ukraine will intensify over the next two to three weeks as Russia continues to refocus its efforts there,” the ministry said in a tweet. “Russian attacks remain focused on Ukrainian positions near Donetsk and Luhansk with further fighting around Kherson and Mykolaiv and a renewed push toward Kramatorsk.”

The southeastern port city of Mariupol has seen some of the heaviest attacks and civilian suffering in the 6-week-old war, but the land, sea and air assaults by Russian forces fighting to capture it have increasingly limited information on circumstances inside the city.