Russia is withdrawing some of its military forces from Ukraine to respond to a Ukrainian offensive into Russian territory, U.S. officials said, the first sign that Kyiv’s incursion is forcing Moscow to rejigger its invasion force.

The officials said the U.S. is still seeking to determine the significance of Russia’s move and didn’t say how many troops the U.S. assesses Russia is shifting. But the new U.S. assessment bolsters claims by Ukrainian officials who said last week’s surprise invasion of Kursk province had drawn Russian forces away from Ukraine, where Moscow’s advantage in manpower and equipment is allowing them to grind forward in several places.