The United States has accused Russia of violating the Minsk ceasefire agreement by deploying more air defence systems in eastern Ukraine.
The U.S. State Department spokeswoman Marie Harf said that the Kremlin was training separatist forces in the conflict-torn region and building up its own forces along the border.
Moscow has not yet responded to the allegations.
Both Ukrainian forces and pro-Russia separatists signed a truce deal, brokered by the West, in Minsk in February.
Ukraine accuses Russia of arming the rebels and sending military support and troops across the border, a claim that Moscow denies.
