US Marines board a C-130J Super Hercules transport aircraft headed to Kandahar as they withdraw from the Camp. AFP

But Austin declined to specifically address whether the US will provide combat air support to the Afghan forces to prevent them or their major cities from being overrun by the Taliban after the pullout is complete. Austin said he would not "speculate on potential actions” but noted that once troops are fully withdrawn from Afghanistan it will be very difficult to provide such support to the Afghans.

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Already, Austin said, the US military is conducting air combat patrols and support from ships in the Gulf, and sending surveillance aircraft to Afghanistan from nations in the Gulf region. And Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff, said that already the "vast majority” of MQ-9 Reaper drone missions are being flown from locations outside Afghanistan.

As of now, Austin said, the US is still looking for options to station troops and aircraft in neighboring countries to make the flight to Afghanistan shorter. But, he said, that effort is still a work in progress and he had no timetable for completion of any negotiations.

The US will focus counterterrorism efforts on Al Qaeda.

Austin and Milley were testifying before the Senate Armed Service Committee on the proposed military budget and were pressed on a wide range of issues, including congressional proposals to change how the military prosecutes major crimes, including sexual assault.

President Joe Biden ordered the complete withdrawal of US troops from Afghanistan by early September, and that pullout is ahead of schedule, the military has said. But many questions surrounding the withdrawal remain unanswered, including how the US will conduct intelligence operations in Afghanistan after the withdrawal, how far it will go to defend the Afghan government and what diplomatic presence it will have in the country.

Austin noted that once the withdrawal is done, the US will focus counterterrorism efforts on Al Qaeda and other terror groups that can conduct attacks against the American homeland.

There have, however, been persistent questions, including from segments of the Afghan government, about whether the US will provide any combat support to the Afghan forces if they are under siege from the Taliban and major cities, such as Kabul, are at risk of being overrun. The Pentagon has suggested that any such mission would not qualify under the parameters set by Biden for counterterror operations, but there is widespread concern that once US and coalition troops have left, the Taliban will increase their attacks and the Afghan government could collapse.

The number of US troops are between 2,500 and 3,500. File photo

As early as this week, the top US commander for the Middle East, Gen. Frank McKenzie, will give Austin a range of military options for securing the US Embassy in Afghanistan and providing counterterrorism support from outside the country once the withdrawal is complete, according to US officials said.

The number of American troops needed for the overall security missions inside Afghanistan will depend on a variety of requirements, and could range from roughly a couple hundred to a bit less than 1,000, officials said. When the pullout officially began on May 1, the number of US troops was between 2,500 and 3,500.

McKenzie is expected to provide options on the amount of aerial surveillance and drones needed to keep an eye on any potential resurgence of Al Qaeda, the Daesh or other militant groups.

And they could range from persistent US overwatch to a more minimal presence. Officials spoke on condition of anonymity to discuss planning details.

On the question of prosecution of sexual assault, both Austin and Milley said they remain open-minded about taking it out of the chain of command. But both also suggested that extending that to all major crimes would require more study, and that any changes to the Uniform Code of Military Justice will take time to enact properly.

The US military has already begun conducting combat operations and surveillance in Afghanistan.

Earlier this year, an independent review commission created by Austin recommended that the prosecution of sexual assaults be shifted to judge advocates reporting to a civilian-led Office of the Chief Special Victim Prosecutor. The independent judge advocates would decide two key legal questions: whether to charge someone and whether that charge should go to a court martial.

Members of Congress are pushing for a similar — and in some respects more expansive — change. Because any such change would amend military law, it will require an act of Congress. A key advocate for the shift has been Sen. Kirsten Gillibrand, a New York Democrat, who now has bipartisan, filibuster-proof support for a bill that would take prosecution decisions out of the chain of command for major crimes, including sexual assault, rape and murder.

On Thursday she pressed Austin and Milley on whether they backed removing all major crimes from the chain of command.

Austin said that so far he and the independent commission have been focused on the sexual assault issue. And Milley expressed reservations about a more expansive change.

"I think the commander is essential to maintaining good order and discipline in the military,” Milley said. "I am very, very open to significant change in the area of sexual assault, sexual harassment. When we get beyond that, at this point my position is I need to study it more... It needs a lot of due diligence before we bundle all the one-year felonies and take them away from the commanders.”

Associated Press