Ukraine’s incursion into a sliver of Russia is likely to make it harder for Moscow to mount a major renewed offensive in Ukraine’s east and is the kind of surprise operation that could eventually impose real costs on the Kremlin, according to U.S. officials.

The Ukrainian strike, and its continued success, could ultimately have strategic significance, though U.S. officials caution that they will need to see how it plays out to draw firmer conclusions. It could also help rebuild sagging morale among Ukraine’s troops and war-weary population, the officials said.

The incursion, into the Kursk region of Russia, stands in stark contrast to Ukraine’s failed counteroffensive in southern Ukraine last summer. This offensive was developed in secret, devised to divert Russian troops away from the front lines in Ukraine and seize territory to use as a bargaining chip.

Ukraine’s monthslong buildup to the counteroffensive played out in the open. The Ukrainians had sought to retake territory but stumbled when they failed to punch through dug-in Russian defenses, which Moscow reinforced as Ukraine trained for the drive. Ukraine also split its forces during that offensive, against American advice, rather than focusing them as they have this month.

The incursion into Russia, in contrast, shows how Ukraine’s army has improved its mechanized warfare skills, techniques that it failed to master a year ago.