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The Pentagon plans to rely on to prevent a resurgence of Al-Qaeda now that US troops have left , but experts and some lawmakers are skeptical about the effectiveness of the so-called "over-the-horizon" strategy.

Announcing the complete withdrawal of in April, President Joe Biden vowed he would not allow a comeback of Al-Qaeda in Afghanistan, where Osama bin Laden hatched the September 11, 2001, attacks on New York and Washington.

Since then, the Pentagon has repeatedly claimed it is capable of keeping Al-Qaeda and Islamic State (IS) militants in Afghanistan in check through from US bases or aircraft carriers.

"Over-the-horizon operations are difficult but absolutely possible," DefenceÌýSecretary Lloyd Austin told the House Armed Services Committee on Wednesday.Ìý

"And intelligence that supports them comes from a variety of sources, and not just US boots on the ground."

Austin's remarks came about two weeks after the Pentagon chief was forced to apologise to the relatives of civilians killed in an August 29 drone strike in Kabul.

The target of the drone strike was suspected IS militants but it ended up killing 10 civilians, including seven children, in what Austin called a "horrible mistake."

It was the latest in a long line of US drone strikes that caused civilian casualties in Afghanistan, becoming one of the most contentious issues over the 20-year war and prompting harsh criticism from Afghans.

In his congressional testimony, Austin declined to publicly divulge much about the Pentagon's "over-the-horizon" plans, telling committee members he could provide more details in a closed classified session.

'Good luck'

A number of experts and lawmakers expressed skepticism about the efficacy of long distance strikes on land-locked Afghanistan, which is thousands of miles (kilometres) from the nearest US base.

"Kill Terrorists in Afghanistan From 'Over The Horizon'? Good Luck," headlined an article by James Holmes, a professor of maritime strategy at the Naval War College.

"Over-the-horizonÌýoperations work well when the battlefield lies within easy reach of sea or air forces," Holmes said in the piece published on the national security website 19fortyfive.com.

"Land-based aircraft flying from Persian Gulf airstrips must detour southward around hostile Iranian airspace, into the Arabian Sea, and northward through Pakistani airspace to strike targets in Afghanistan," said Holmes, a former US Navy officer.

"Carrier aircraft have it easier from a distance standpoint since their mobile airfield can linger in the Arabian Sea," he added.

"But even so, the Afghan capital of Kabul lies close to 700 miles from the closest point along the Pakistani seacoast," Holmes said. "Inflight refuelling will be a must."

'A fiction'

Mike Waltz, a Republican lawmaker from Florida, accused Biden and Austin of peddling a "fiction" when it came to "over-the-horizon" capability.

Unlike in Iraq, where US troops fought IS with Iraqi government forces, or Syria, where Americans partnered with Kurdish fighters, the United States does not have any allies on the ground in Afghanistan or any nearby bases, Waltz said. Ìý

"Those drones have to fly all the way around Iran, all the way up Pakistan and lose 70 to 80 percent of their fuel before they even get anywhere near a target," said Waltz, a former US Army Green Beret who served in Afghanistan.

"The president of the United States is selling this country a fiction that we can do over here with nothing," Waltz said, pointing to Afghanistan on a map, "what we're doing over here (in Iraq and Syria) with neighbouring bases access, with allies on the ground and with ocean access."

"That is a fiction that you all need to own," he added.

In the early 2000s, the United States had military bases in Uzbekistan, Tajikistan and Kyrgyzstan, but it is no longer present in Central Asia, which Russia considers its sphere of influence.

General Mark Milley, the chairman of the US Joint Chiefs of Staff, told lawmakers that he met recently in Europe with his Russian counterpart, General Valery Gerasimov.

"In the main, we're not asking permission -Ìýnegotiating, I guess, is the word," Milley said.

"President (Vladimir) Putin and President Biden had a conversation and I was following up on that conversation," he said.Ìý

Andy Kim, a Democratic lawmaker from New Jersey, asked Austin, the Pentagon chief, whether US overflights of Afghanistan were legal.

"Yes," Austin replied, adding that he would provide further details in a classified setting.

The this week accused the United States of violating international law with drone flights over Afghan territory and warned of "negative consequences" if they continued.