WASHINGTON, Aug. 13 - US Secretary of State John Kerry called his Russian counterpart Sergei Lavrov on Thursday to express "grave concern" about increased separatist attacks in Ukraine, a senior State Department official said, AFP reported.

Washington has long accused Russia of fomenting unrest in a breakaway region of eastern Ukraine and has urged Moscow to help implement the terms of a shaky ceasefire deal while pushing for a political solution to the conflict.