An open-air market in the Ukrainian coastal city of Mariupol came under rocket fire on Saturday, Reuters reports. According to city officials, at least 30 people were killed.

Mariupol, under Ukrainian control, is strategically important as it lies between mainland Russia and the Russian-annexed Crimea. "Heavy fighting in the region in the autumn raised fears that Russian-backed separatist forces would try to establish a land link between Russia and Crimea," the Associated Press reports.

On Friday, rebel leader Aleksandr Zakharchenko said that separatists intended to go on the "offensive," the New York Times reported. The day before, a mortar attack on separatist-held Donetsk reportedly killed 13 people. The AP has shared graphic photos of the incident.

The cease-fire devised by Vladimir Putin and signed in September after five months of fighting appears to have collapsed. "It was pure illusion that peace could be achieved now," Enrique Menendez, who runs a humanitarian relief operation in eastern Ukraine told the Times. "The only real surprise is that the fighting started in the winter instead of the spring."

[Photo credit: AP Images]