NATO Secretary-General Anders Fogh Rasmussen will visit Kyiv on August 7, Ukraine's Foreign Ministry announced.

The visit, at the invitation of Ukrainian President Petro Poroshenko, is intended to discuss an upcoming meeting on the NATO-Ukraine partnership.

Earlier on August 6, NATO said a Russian force buildup near the Ukrainian border amounts to some 20,000 troops.

NATO said Moscow could use the excuse of a humanitarian or peacekeeping mission to send them into Ukraine.

Moscow on August 4 announced what it said were military training exercises near the border with Ukraine.

The Ukrainian government says fierce clashes with pro-Russian rebels had left 18 soldiers dead over the past 24 hours.