Russia has readied around 20,000 troops on Ukraine's eastern border and may invade under a so-called humanitarian or peace-keeping mission as an excuse, NATO warned on Wednesday.

The Ukraine-Russia conflict, NATO spokeswoman Oana Lungescu said in a statement, had escalated into "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.

Moscow has already denied the accusations that it has armed and supported rebels fighting Ukrainian forces in eastern Ukraine, but has made a show of announcing its new military exercises taking place this week, including placing bombers and warplanes near the border on Monday.

The Kremlin over the past year has amassed and removed troops, beginning with 40,000 by the border, and then reducing the number down to less than 1,000. But after weeks of increased conflict, according to a NATO military officer who spoke anonymously to Reuters, Russia latest additions include tanks, infantry, artillery, air defense systems, logistics troops, special forces, and various aircraft. 

The conflict has escalated since the Malaysia Air Flight 17 disaster in July. As a result, the European Union and the U.S. have placed tough sanctions against Russia, making this the biggest confrontation between Russia and the West since the Cold War.