An agreement to avert wider conflict in Ukraine was uncertain on Monday, with pro-Moscow separatist gunmen showing no sign of surrendering government buildings they have seized.

The separatists have occupied government buildings across swathes of eastern Ukraine over the past two weeks

U.S. and European officials say they will hold Moscow responsible and impose new economic sanctions.

U.S. Vice President Joe Biden arrived in Kiev, where he is expected to announce a package of technical assistance.

The visit is likely to be more important as a symbol of support than for any specific promises Biden makes in public.

“He will call for urgent implementation of the agreement reached in Geneva last week while also making clear … that there will be mounting costs for Russia if they choose a destabilizing rather than constructive course in the days ahead,” a senior administration official told reporters.

Russia, Ukraine, the European Union and the United States signed off on the agreement in Geneva on Thursday designed to lower tension in the worst confrontation between Russia and the West since the Cold War.

The agreement calls for occupied buildings to be vacated under the auspices of envoys from the Organization for Security and Cooperation in Europe.

But no sooner had the accord been signed than both sides accused the other of breaking it, while the pro-Moscow rebels disavowed the pledge to withdraw from occupied buildings.