Russia has amassed around 20,000 combat-ready troops on Ukraine's eastern border and could use the pretext of a humanitarian or peacekeeping mission to invade, NATO said on Wednesday, Aug 6, according to Reuters.

Insisting that the conflict in Ukraine was fueled by Russia, NATO said in a statement that the troop build-up had further escalated "a dangerous situation".

"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground – and that is of great concern. Russia has amassed around 20,000 combat-ready troops on Ukraine’s eastern border," NATO spokeswoman Oana Lungescu said in an emailed statement.

NATO was concerned that Moscow could use "the pretext of a humanitarian or peace-keeping mission as an excuse to send troops into Eastern Ukraine", she said.

Moscow denies Western accusations that it has armed its supporters who are fighting Ukrainian forces in the country’s east. Russia announced new military exercises to take place all this week, involving bombers and warplanes on Monday in a show of strength near the border with Ukraine.

Polish Prime Minister Donald Tusk said on Wednesday the threat of a direct intervention by Russia's military in Ukraine has risen over the last couple of days.

NATO said early this year that Russia had amassed some 40,000 troops close to the Ukraine border. By June, the number had dropped to less than 1,000, but then Russia started building the force up again.

A NATO military officer, speaking on condition of anonymity, said that early this month Russia had significantly increased the number of troops near the Russian border.

The roughly 20,000-strong troop presence included tanks, infantry, artillery, air defense systems, as well as logistics troops, special forces, and various aircraft, Reuters quoted the officer as saying.