Secretary of State John Kerry urged Syrian government forces Saturday to stop bombarding the city of Aleppo as it moves in to retake it.

Kerry said the priority was “a durable, nationwide cessation.” The UN envoy for Syria and the lead opposition negotiator have urged the U.S. and Russia to step in and take control of peace talks as a fragile truce hangs in the balance.

The truce between Syrian President Bashar Assad and the U.S.-backed moderate rebels was called in February with the help of the U.S. and Russia.

This week, fierce bombing has ramped up with heavy civilian casualties, including the bombing of a Doctors Without Borders hospital that killed one of the last pediatricians left in the country. However, Russia has resisted calls to pressure Assad to stop his bombing campaign.