Muddy terrain and the need for more troops on the ground make any large-scale Russian invasion of Ukraine unlikely until January at the earliest, military analysts in Moscow and the West say, creating a window of diplomacy to steer President Vladimir Putin away from a war.

Those judgments come as a combination of open source reports and images gives growing credence to U.S. intelligence assessments of a build-up of soldiers and equipment toward Ukraine’s borders. Ukrainian military maps also show the changing distribution of Russian forces.

The U.S. has shared the intelligence and its own maps with some NATO allies, warning that Russia could be weighing a large-scale incursion. U.S. officials said in their briefings that Putin’s intentions remain unknown, but any operation, should he decide to act, would likely involve double the number of battle groups currently in position and could come in the early months of next year.