All parties to the conflict in Donbas should stop using heavy weapons and return to observing the ceasefire, OSCE Secretary General Helga Schmid said.

"I call on the sides to withdraw heavy weapons from the security zone, adhere to the ceasefire and protect civilians," she said on Twitter Sunday.

Schmid noted that the OSCE observers recorded "increased use of heavy weapons proscribed by the Minsk agreements."

"Putting the lives of civilians at risk is unacceptable," she stressed.