Whatever Mr. Putin’s intentions, American officials say they are rallying European allies for coordinated sanctions that would snap into place as soon as military action began. It is not clear, however, that the threat of such penalties would impress Mr. Putin, who has noted that Russia has lived with Western sanctions for years.

American and European officials are also continuing to monitor cyberattacks and disinformation campaigns that they say may be preparing the battlefield for any action. The senior administration official noted that such action has “often been done in advance these of sorts of incursions in the past.” The disinformation campaign has attempted to create a narrative that Ukraine is the country provoking a conflict.

In another sign of Western engagement, Valery V. Gerasimov, Russia’s top military commander, spoke with his British counterpart, Sir Tony Radakin, on Thursday, and with Mark A. Milley, the chairman of the Joint Chiefs of Staff, on Wednesday. But in a move underscoring the continuing tensions, Russia on Thursday announced snap paratrooper exercises in and around Crimea, the Black Sea peninsula annexed from Ukraine in 2014.