Ukrainian forces mounted a major attack in the southern region of Zaporizhzhia on Thursday, taking the offensive against the invading Russians in multiple places in the east and south, but there was no indication of a breakthrough in an operation that carries high stakes for Kyiv and its Western allies.

Three senior U.S. officials, as well as military analysts, said that a long-awaited major Ukrainian counteroffensive appeared to be underway, after months spent mobilizing and training new units, and arming them with advanced Western weapons. A strong indication, they said, was the Ukrainians’ use in combat on Thursday of German Leopard tanks and American Bradley fighting vehicles.

“It appears some of the new brigades Ukraine stood up for this counteroffensive have been committed, which indicates the counteroffensive is underway,” said Rob Lee, a senior fellow with the Foreign Policy Research Institute. “Ukrainian forces have made some tactical gains and sustained losses.”

The United States and other Ukrainian allies trained and equipped the nine brigades that were designed to lead Ukraine’s counteroffensive to recapture Russian-occupied land. Ukraine is widely expected to mount a multipronged assault along a front that arcs for hundreds of miles, focusing on a swath of the Zaporizhzhia and Donetsk regions. Ukraine’s plans target specific areas to try to break through Russian lines, but can adjust to concentrate on those thrusts that prove most successful, U.S. officials said.