This week, heavy shelling and gunfire returned to the war-torn front lines of eastern Ukraine, putting an already fragile ceasefire between Ukrainian government and Russian-backed rebel forces in danger of total collapse.

At least four civilians have been reported killed as shells hit apartment blocks in the government-held town of Avdiivka. In the nearby rebel stronghold of Donetsk, shelling began late Sunday and continued into the night, hitting stores in the city center. It was the first time shelling had hit central Donetsk, one of Ukraine’s largest cities, since an internationally-brokered ceasefire took effect in February.

The renewed violence comes as Ukrainian and NATO officials warn of a new large-scale offensive by rebel forces as material support continues to arrive from Russia. Kiev has voiced concern that Russian-backed rebels may attempt to capture the key port city of Mariupol and carve a land bridge to Russian-controlled Crimea.

Both Ukrainian and rebel forces have accused each other of responsibility for this week’s hike in violence. Ukrainian military official Serhiy Halushko said the army had intercepted rebel radio traffic over plans to shell Donetsk and blame Kiev. Rebels, however, accuse Kiev of seeking to shatter the ailing ceasefire.

International observers noted that shelling originated from within the city, toward government-held positions, which then returned fire into the city.