U.S. President Joe Biden has ordered B-52 bombers and Spectre gunships to target Taliban positions in Afghanistan "in an effort to protect allies," as the militants advance towards key cities, local TV channel Tolo News reported on Saturday.

U.S. B-52 Stratofortress bombers launched airstrikes on the Taliban forces in Sheberghan, the capital city of Jawzjan province in northern Afghanistan, Afghan Defense Ministry spokesperson Fawad Aman said.

U.S. Air Force confirmed the operation, saying the U.S. forces "have conducted several airstrikes in defense of our Afghan partners in recent days," U.S. Air Force Major Nicole Ferrara, a U.S. Central Command spokesperson, told media via email.

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Taliban capture two Afghan provincial capitals within 24 hours

The U.S. Embassy in Kabul on Saturday also urged U.S. citizens to leave Afghanistan immediately "using available commercial flight options" in a statement published on its website that cited "the security conditions and reduced staffing."

Starting from May 1, following the announcement made by Biden, the last U.S. ground forces are scheduled to leave Afghanistan by August 31. Meanwhile, the Taliban have taken control of vast tracts of rural Afghanistan since early May when they launched a series of offensives to coincide with the start of the final withdrawal of foreign troops.

On Saturday, the Taliban announced that they had seized the southwestern province of Nimruz and the northern province of Jawzjan within 24 hours.

(With input from agencies)