Tensions over Russia’s military buildup near Ukraine are entering a potentially decisive week, with the U.S. warning an invasion may be imminent and President Vladimir Putin accusing America of failing to meet his demands.

U.S. national security adviser Jake Sullivan, who on Friday cited the risk that Russia will attack or seek to ignite conflict within Ukraine this week, told CNN on Sunday there’s "a distinct possibility that there will be major military action very soon.”

The U.S. and its European allies have been pressing Putin for weeks to pull back an estimated 130,000 troops massed near Ukraine, but a frenzy of diplomatic activity — including key leaders cycling through Moscow for talks with Putin — has failed to produce an off-ramp.