The U.S. is concerned about movement of heavy weapons and tanks as the fight is escalated in the eastern Ukraine between the Ukrainian forces and pro-Russian separatists. 

White House National Security Council spokesperson Bernadette Meehan said Sunday that numerous reports, including from the Special Monitoring Mission of the Organization for Security and Cooperation in Europe, has revealed that pro- Russian separatists are moving large convoys of heavy weapons and tanks to the front lines of the conflict. 

"Any attempt by separatist forces to seize additional territory in eastern Ukraine would be a blatant violation of the Minsk agreements," she said in a statement. 

The Minsk agreement was signed on Sept. 5 after extensive talks between Russia, Ukraine and pro-Russian separatists under the auspices of the Organization for Security and Co-operation in Europe (OSCE).

The agreement implemented an immediate cease-fire that failed to stop fighting in the eastern regions of the country.

Meehan also called on Russia to keep the commitments it made in Minsk, including ending its military supply to the separatists and the withdrawal of all of its troops and weapons from Ukraine. 

"Russia must enable the restoration of Ukrainian sovereignty along the Ukrainian side of the international border, to be monitored by the OSCE, and facilitate the release of all hostages," she said. 

The stalemate in Ukraine between the government and the separatists has left more than 4,000 dead and over 9,000 others wounded since April, a month before Russia annexed Crimea in March following a referendum vote held in the peninsula that was not recognized by Ukraine or the West.

Anadolu Agency