Concern is growing that the long-simmmering conflict in eastern Ukraine could erupt into widespread fighting following a sharp increase in violence along the front line.

Kiev's top diplomat Dmytro Kuleba will meet NATO chief Jens Stoltenberg as US Secretary of State Antony Blinken also heads to Brussels for talks.

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

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

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.

Blinken warned in an interview on Sunday of "consequences" if Russia acts "aggressively" towards Ukraine.

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.

Ukraine has urged NATO to step up Kiev'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.