Mariupol, Ukraine (CNN)Less than two days into a ceasefire in eastern Ukraine, deadly violence erupted once again between the military and pro-Russian separatists, casting further doubt about whether the peace plan will hold.

Five Ukrainian security forces were killed and 25 were wounded in fighting with the separatists, Ukrainian defense spokesman Andriy Lysenko said.

He said at least 129 violations of the ceasefire have already taken place -- ranging from small arms fire to mortar attacks. But pro-Russian separatists have also blamed Ukraine for violations.