The White House expressed grave concern Sunday at reports of Russian military reinforcements in eastern Ukraine, warning that any separatist efforts to seize more territory would be a "blatant violation" of a ceasefire agreement.

"We are very concerned by intensified fighting in eastern Ukraine, as well as numerous reports... that Russian backed and supplied separatists are moving large convoys of heavy weapons and tanks to the front lines of the conflict," National Security Council spokeswoman Bernadette Meehan said in a statement.