According to a U.S. official, Russian troops near Ukraine have moved into "attack posts."

According to a U.S. official, Russia moved long-range rocket launchers and artillery into firing position, potentially threatening Ukraine.

According to a U.S. official, Russian troops near Ukraine have moved into "attack posts."

According to an official, some Russian units have begun to move into "attack points" after leaving their assembly areas. These are the bumper-to–bumper formations visible in satellite photos. This is a significant change from Sunday's situation, when some units left their assembly areas but hadn't yet taken what could be considered attack positions.

According to the official, the U.S. believes Russia will attack Ukraine before the end of this week. However, it isn't yet clear what form it will take.

The U.S. will have a dramatically decreased understanding of Russian events once they begin. The Americans will not have troops on the ground or reconnaissance planes flying over the air. Russian cyber attacks and electronic warfare are likely to obstruct communications.

According to U.S. officials, Russia has now 80% of the forces required for a full-scale invasion and the rest are on their way. More than 100,000 Russian troops have been assembled along Ukraine's borders - to the east in Russia and north in Belarus.

National security advisor Jake Sullivan warned Sunday Morning on "Face the Nation," that Russian President Vladimir Putin could issue orders for an invasion "essentially at any moment."

"We have witnessed over the past 10 days a dramatic acceleration of the build-up Russian forces and the dispositions of those forces so that they could launch a militarily action essentially any time," Sullivan stated. They could do it this week, but they still need to get the go-order from Putin.

Sullivan acknowledged there is a possibility that Russia could launch a false flag attack, possibly in the Donbas area of Ukraine. He said that the U.S. was "watching very closely." Sullivan stated Sunday that Russian intelligence services could "conduct some type of attack against Russian proxy forces or Russian citizens in eastern Ukraine and then blame it onto the Ukrainians."

Monday's developments in Ukraine are being presented by Sullivan to congressional leaders.

Saturday's warning by President Biden to Russian President Vladimir Putin was that the U.S. would "respond decisively" and "impose swift and severe penalties" if Russia invades Ukraine.