BRUSSELS — NATO’s top commander said on Wednesday that the 40,000 troops Russia has within striking distance of Ukraine are poised to attack on 12 hours’ notice and could accomplish their military objectives within three to five days.

President Vladimir V. Putin of Russia told Chancellor Angela Merkel of Germany on Monday that the Kremlin was beginning to withdraw troops from the border area near Ukraine.

But the NATO commander, Gen. Philip M. Breedlove, said in an interview with The New York Times that so far only a single battalion, a force of 400 to 500 troops, was on the move and that NATO intelligence could not say whether it was actually being withdrawn.

“What we can say now is that we do see a battalion-size unit moving, but what we can’t confirm is that it is leaving the battlefield,” said General Breedlove, of the United States Air Force. “Whether that movement is aft to a less belligerent configuration or returning to barracks, we do not see that.”