Ukraine's foreign minister will hold talks on Tuesday at NATO headquarters as America's top diplomat also visits Brussels amid a rise in tension over increased Russian military activity.   

Concern is growing that the long-simmering conflict in eastern Ukraine could erupt into widespread fighting following intensified clashes with Moscow-backed separatists and signs of a Russian troop build-up on its borders. 

NATO said Kiev's top diplomat Dmytro Kuleba will meet alliance chief Jens Stoltenberg as Washington announced US Secretary of State Antony Blinken was also heading to Brussels for talks.

Blinken and Stoltenberg spoke by phone Monday ahead of his visit to discuss issues "including the immediate need for Russia to cease its aggressive military buildup along Ukraine's borders and in occupied Crimea," the state department said. 

America's top diplomat warned in an interview on Sunday of "consequences" if Russia acts "aggressively" towards Ukraine. 

Kiev has accused its giant neighbour Russia of amassing thousands of military personnel on its eastern border as well as on the annexed Crimean peninsula.

The White House said last week the number of Russian troops at the border with Ukraine was now greater than at any time since 2014, when the conflict erupted after Moscow seized Crimea from Kiev.

Reports of the build-up have swirled as deadly clashes flared with Russia-backed separatists in the east of the country. 

Ukraine's military said Monday that two of it soldiers were killed in the latest bloodshed. 

The Kremlin, which has not denied the troop movements, said it was not moving towards war with Ukraine -- but also stressed it "will not remain indifferent" to the fate of Russian speakers in the conflict-torn region.

Kiev has urged NATO to step up Ukraine's push for membership as it seeks to deter any aggression by Russia -- but the alliance remains wary about speeding up plans to allow in the ex-Soviet nation.