WASHINGTON — U.S. experts on Russia see a growing danger the crisis in Ukraine that has already taken thousands of lives could explode, although the latest cease-fire agreement has partially held.

U.S. and Western European moves to isolate Russia and damage its economy for its seizure of the Crimean peninsula and support of separatists in eastern Ukraine against the Western-backed central government have not forced President Vladimir Putin to back down.

The State Department says Russia is beefing up its troops and weapons along the Ukrainian border. Meanwhile, the U.S. has dispatched 300 American military trainers to work with the Ukraine army, and other NATO members are adding hundreds more.

Experts say that’s a fuse of a much wider conflict waiting to be lit after a year of battle that the U.N. says has taken at least 6,100 lives.

Andrew Weiss, Russia expert at the Carnegie Endowment for International Peace, said an egregious action by Russian forces in eastern Ukraine or an incident between U.S. and Russian forces could cause an escalation. He said a recent near-collision of U.S. and Russian military jets over the Baltics highlighted the risk.