In exchange for the release of Russian prisoners of war, Ukraine’s president said arduous negotiations were ongoing to evacuate “a substantial number” of wounded soldiers from a besieged steelworks in the important southeastern port of Mariupol.

Hundreds of Ukrainian fighters are still holding out at the Azovstal steelworks after weeks of Russian bombing. Mariupol, which has seen the most intense combat in nearly three months of war, is now in Russian control.

Russian advances around Ukraine have slowed and in some instances reversed due to fierce Ukrainian opposition, which military analysts say President Vladimir Putin and his generals failed to expect when they launched the invasion on Feb. 24.