▶ Watch Video: Russia readies for war in Ukraine as U.S. weighs response

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

Some Russian units have left their assembly areas — the bumper-to-bumper formations seen in satellite photos — and are beginning to move into “attack positions,” according to the official. This movement marks a change since Sunday, when some of the units had left the assembly areas but had not yet taken what could be viewed as attack positions.

The U.S. believes Russia will attack Ukraine by the end of the week, although it’s not yet certain what form it will take, the official said. 

Once the Russian attack begins, U.S. knowledge of what is happening will dramatically decrease. Americans will have no troops on the ground and no reconnaissance planes over the air, and Russian cyber attacks and electronic warfare will blot out communications.  

On Saturday, President Biden warned Russian President Vladimir Putin that the U.S. will “respond decisively and impose swift and severe costs,” should Russia invade Ukraine. 

— This is a developing story.