The Pentagon cannot confirm reports that Russia plans to invade Ukraine on Wednesday, spokesperson John Kirby said Sunday.

"I'm not in a position to confirm those reports," Kirby said during an interview on "Fox News Sunday," adding that the United States believes that a "major military action could happen any day now."

Since 2014, Russia has been supporting separatist rebels in eastern Ukraine, a policy it has maintained for the past eight years.

In response to Russia's actions, NATO enhanced its presence in the eastern bloc, with four multinational battalion-size battle groups deployed to Estonia, Latvia, Lithuania and Poland on a rotational basis.