Ukrainian forces staged an overnight ambush on a Russian convoy 25 miles inside the international border in Russia’s Kursk province, as the Kremlin declared a federal emergency and said it was transferring extra forces to try to snuff out a four-day incursion that has badly damaged its credibility.

A video circulated by Russian military bloggers showed a destroyed convoy, with bodies just visible inside some trucks, on the E38 east-west highway at Oktyabrskoe, a location far deeper inside Russia than any previously confirmed fighting since Ukraine’s forces crossed the border on Tuesday.

Commentators said the attack, reminiscent of Ukrainian attacks on Russian troops besieging Kyiv in the first weeks of the war, demonstrated an effective hit-and-run strategy, but the incursion appeared likely to draw an escalating response from the Kremlin, and its overall outcome remains profoundly uncertain.