Two Ukrainian servicemen were killed by pro-Russian separatists in eastern Ukraine in the past 24 hours, Ukraine’s military announced on Tuesday, adding to already escalating tensions.

Kyiv says Moscow has been building up its troops near Ukraine’s eastern borders in recent weeks, and reports of separatist attacks on Ukrainian government forces have increased significantly.

NATO has voiced its concerns about a Russian military build-up in the Donbass region, where Ukrainian troops have battled Russian-backed separatist forces in a conflict Kyiv estimates has killed 14,000 people since 2014.

Russia and Ukraine have long accused each other of failing to enforce a peace deal over the region.

On Monday, the Kremlin said Russian military movements near its shared border with Ukraine posed no threat to Kyiv or anyone else and that Moscow moved troops around its country as it deemed fit.

The United States, the European Union, NATO countries and Britain have expressed support for Ukraine.

Washington finds reports of Russian military movements on Ukraine’s border “credible,” has asked Moscow to explain the “provocations” and is ready to engage on the situation, the U.S. State Department said on Monday.