Weeks later, Mr Putin withdrew some 10,000 troops near Ukraine. But US officials said those troops were not part of the force Mr Putin appeared to be gathering for a possible invasion in January or February.
Understand the escalating tensions over Ukraine
Still, intelligence officials believed more troops would be built in December. A senior government official said the Russian deployment continued, but at a slower pace than in early December.
Currently, Russia has just under 60 tactical battalion groups on the ground, or somewhere between 85,000 and 100,000 troops, according to US officials. Those troops have been holding drills and exercises, which show that the Russian troops are at the highest level of preparedness.
In addition to building up aviation assets, the Russian government has ordered more units specializing in logistics. While aviation assets would be critical to protecting ground forces during the invasion, the logistics units would be needed to support the ground forces in the event that Mr Putin orders his troops across the border.
More than 150 US military advisers are in Ukraine, trainers who have worked for years from the training ground near Lviv, in the west of the country, far from the front lines. The current group includes Special Operations troops, primarily Army Green Berets, as well as National Guard trainers from Florida’s 53rd Infantry Brigade Combat Team.
Military advisers from about a dozen allied countries are also in Ukraine, US officials said. Several NATO countries, including Britain, Canada, Lithuania and Poland, have regularly sent training troops to the country.
In the event of a large-scale Russian invasion, the United States plans to expel its military trainers quickly. But it’s possible some Americans could stay to advise Ukrainian officials in Kiev, the capital, or provide frontline support, a US official said.