Lo ha affermato la NATO:

(Reuters) – Russia has massed around 20,000 combat-ready troops on Ukraine’s border and could use the pretext of a humanitarian mission to invade, NATO said on Wednesday, its starkest warning yet that Moscow could soon mount a ground assault against its neighbor.
With fighting escalating and rebels losing ground in the weeks since a Malaysian airliner was shot down over separatist-held territory, Russia has announced military exercises this week in the border region.
“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.
Moscow could use “the pretext of a humanitarian or peace-keeping mission as an excuse to send troops into Eastern Ukraine”, she said. […]

The latest Russian troop build-up on the border is not the first time Moscow has concentrated forces there: NATO estimated Russia had as many as 40,000 troops in place earlier in the crisis before President Vladimir Putin pulled them back in June.
But the government’s advance since then may be prompting new action from the Kremlin, after months in which state-controlled Russian media have mounted a sustained campaign of anti-Ukrainian agitation and nationalist pride focused on Crimea.
Since March, Putin has vowed to use military force to protect Russian-speaking “compatriots” across the former Soviet Union. He branded southern and eastern Ukraine “New Russia”, a name the rebels took up as the banner for their movement.
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.
A NATO military officer, speaking on condition of anonymity, said that Russia’s recent build-up at the border included tanks, infantry, artillery, air defense systems, logistics troops, special forces, and various aircraft.
The confrontation between Moscow and the West has turned into a trade war since the United States and EU tightened their sanctions to hit Russia’s oil, defense and finance sectors.
President Vladimir Putin ordered his government on Tuesday to work out retaliatory sanctions, but said the government should make sure consumers don’t get hit. […]