The United States will keep extra forces in Europe to bolster NATO defenses and monitor new movements of Russian tanks in east Ukraine as a cease-fire deadline approaches, officials said Thursday.

Under the terms of the peace agreement, Ukrainian forces and Russian-backed separatists are to reach a full cease-fire by 12:01 a.m. Sunday followed by the complete withdrawal of all heavy weapons 14 days after the cease-fire. The terms were agreed to late Wednesday and into early Thursday even as reports came in that Russia moved a column of 50 tanks into east Ukraine.