Russian forces pounded the Ukrainian port city of Mariupol on Saturday, shelling its downtown as residents hid in an iconic mosque and elsewhere to avoid the explosions. Fighting also raged in the outskirts of the capital, Kyiv, as Russia kept up its bombardment of other cities throughout the country.

Mariupol has endured some of Ukraine’s worst punishment since Russia invaded. Unceasing barrages have thwarted repeated attempts to bring food, water and medicine into the city of 430,000 and to evacuate its trapped civilians. More than 1,500 people have died in Mariupol during the siege, according to the mayor’s office, and the shelling has even interrupted efforts to bury the dead in mass graves.

Talks aimed at reaching a cease-fire again failed Saturday, and while the U.S. announced plans to provide another $200 million to Ukraine for weapons, a senior Russian diplomat warned that Moscow could attack foreign shipments of military equipment.

Ukrainian President Volodymyr Zelenskyy accused Russia of employing “a new stage of terror” with the alleged detention of a mayor from a city west of Mariupol.