Russian leader Vladimir Putin "continues to add forces along that border with Ukraine and in Belarus, even just over the course of the weekend, he's well north of 100,000," spokesman John Kirby told CNN.

"It's not just the numbers," Kirby continued. "It's combined arms capabilities that he has available to him from everything from armor to infantry to special operations to cyber and air and missile defense."

The United States has warned that a Russian invasion of Ukraine could be imminent, and their Western allies have threatened a crippling package of economic sanctions in response.

Russian Defense Minister Sergei Shoigu announced on Monday the end of some of Moscow's massive military drills in Russia and Belarus, though he told Putin others were ongoing.

Earlier, Russian Foreign Minister Sergei Lavrov told Putin that there was a "chance" of reaching an agreement on security with the West, in what appeared to be a possible climbdown amid the raging tensions.

"Our possibilities are far from being exhausted, they certainly should not continue indefinitely, but I would suggest continuing and ramping them up," Lavrov said in a carefully choreographed meeting with Putin.

Despite the signs of easing tensions, Kirby reaffirmed that the Kremlin leader has all the capabilities to launch a vast, conventional military offensive in Ukraine at any time, or even a smaller attack to destabilize the country.