WASHINGTON (BLOOMBERG) - US military aircraft have been hitting ground targets in Afghanistan in an effort to protect allies, according to the US Central Command, as the Taleban makes rapid advances in the void left by withdrawing American and North Atlantic Treaty Organisation troops.

"US forces have conducted several air strikes in defence of our Afghan partners in recent days," Major Nicole Ferrara of the US Air Force, a spokesman for the US Central Command, said by e-mail.

She declined to provide specifics on the aircraft involved.

US President Joe Biden ordered B-52 bombers and AC-130 Spectre gunships to strike enemy fighters advancing towards Kandahar and other cities, the Daily Mail reported on Saturday (Aug 7).

A National Security Council spokesman referred questions to the US Defence Department.

The Taleban will avenge US air strikes with "full strength", spokesman Qari Yousef Ahmadi said in an e-mail statement on Sunday, adding that the US targeted civilian infrastructure such as hospitals and schools in the southern Helmand province.

The last US ground forces are scheduled to leave Afghanistan by Aug 31. The US Embassy in Kabul on Saturday urged US citizens to leave Afghanistan immediately "using available commercial flight options", according to a statement on its website that cited "the security conditions and reduced staffing".

The Taleban has been gaining territory and taking over Customs posts at Afghan border crossings, seizing much of the government's revenue.

It now controls half of the country's 419 districts, and the militants are putting pressure on the provincial capitals, General Mark Milley, chairman of the US Joint Chiefs of Staff, said last month.

Afghan security forces are consolidating around the cities as Taleban fighters attempt to isolate those population centres, he said.