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Russian forces pounded frontline cities in eastern Ukraine with air strikes and artillery attacks, while U.S. officials stepped up efforts to locate the source of a leak of classified U.S. documents, including those on Ukrainian counter-offensive plans.

The Russians pressed on with their offensive in the eastern Donetsk region where several cities and towns came under heavy bombardment, Ukraine's general staff said on Tuesday.

Ukrainian forces repelled several attacks, it said, as the Russian military kept up its effort to take control of Bakhmut.

A top Ukrainian commander accused Moscow of using "scorched earth" tactics.

"The enemy switched to so-called scorched earth tactics from Syria. It is destroying buildings and positions with air strikes and artillery fire," Colonel General Oleksandr Syrskyi, commander of Ukraine's ground forces, said of Bakhmut.

The battle for the small and now largely ruined city on the edge of a chunk of Russian-controlled territory in Donetsk has been the bloodiest of the 13-month war as Moscow tries to inject momentum into its campaign after recent setbacks.

Both sides have suffered heavy casualties in the Bakhmut fighting, but Syrskyi said: "The situation is difficult but controllable."

The head of the Moscow-controlled part of Donetsk, Denis Pushilin, said Russian forces now held 75% of the city, though he cautioned it was too early to talk about Bakhmut's fall.

Moscow's military was also targeting the city of Avdiivka.

"The Russians have turned Avdiivka into a total ruin," said Pavlo Kyrylenko, Donetsk's regional governor, describing an air strike on Monday that destroyed a multi-storey building.

"In total, around 1,800 people remain in Avdiivka, all of whom risk their lives every day."

In Chasiv Yar, the first major town to Bakhmut's west, few buildings remain intact and those queuing for food and other aid do not even flinch at the sound of artillery.

"It used to be scarier, but now we have got used to it," said 50-year-old humanitarian volunteer Maksym. "You don't even pay attention," he added, his words nearly drowned out by the sound of explosions.

As the battles ground on, U.S. broadcaster CNN said Ukraine was forced to amend some military plans ahead of its long-anticipated counter-offensive because of the leak of dozens of secret documents.

U.S. officials are trying to trace the source of the leak, reviewing how they sharesecrets internally and dealing with the diplomatic fallout.