Ukrainian forces mounted a major attack overnight on Thursday in the southern region of Zaporizhzhia, as Ukraine’s army went on the offensive on multiple fronts in an operation that carries high stakes for Kyiv and its western allies.

A senior US official said on Thursday that the attack appeared to be a main thrust of a much-anticipated Ukrainian counteroffensive. Russia claimed its forces had withstood a Ukrainian assault involving tanks and armoured vehicles, saying it had so far thwarted Ukraine’s attempts to recapture land. Kyiv remained quiet during the intensified fighting.

For months, Kyiv officials have been mobilising new units, gathering weapons and training for what its leaders have billed as a major counteroffensive aimed at pushing Russian forces back and retaking occupied territory.

In recent days, Russia has reported that Ukrainian forces have launched attacks on front lines in the east and south. The fighting in the east, in the Donetsk region, prompted American authorities this week to say that the counteroffensive may have begun.

Taken together, these attacks suggest Ukrainian forces are increasingly on the offensive, though it remains unclear whether the assaults on Russian lines are preludes to a larger push or mark the start of the much-anticipated counteroffensive Kyiv’s generals have said they have planned.

US officials, including the one who spoke on Thursday, requested anonymity to discuss operational details.

Michael Kofman, the director of Russian studies at CNA, a research institute in Arlington, Virginia, said that Ukraine had amassed western-provided Leopard tanks and American-made Bradley fighting vehicles near Zaporizhzhia, in a possible sign that a major assault there was underway.