US Defense Secretary Lloyd Austin said on Thursday the al-Qaida extremist group that used Afghanistan as a staging base to attack United States 20 years ago may attempt to regenerate there following an American withdrawal that has left the Taliban in power.

He said the United States is prepared to prevent an al-Qaida comeback in Afghanistan that would threaten the United States.

“The whole community is kind of watching to see what happens and whether or not al-Qaida has the ability to regenerate in Afghanistan,” he said. “The nature of al-Qaida and (the Islamic State group) is they will always attempt to find space to grow and regenerate, whether it’s there, whether it’s in Somalia, or whether it’s in any other ungoverned space. I think that’s the nature of the organization.”

The Taliban had provided al-Qaida with sanctuary while it ruled Afghanistan from 1996 to 2001. The U.S. invaded and overthrew the Taliban after it refused to turn over al-Qaida leaders following the Sept. 11, 2001 attack on the United States.

“We put the Taliban on notice that we expect them to not allow that to happen,” Austin said, referring to the possibility of al-Qaida using Afghanistan as a staging base in the future.

In a February 2020 agreement with the Trump administration, Taliban leaders pledged not to support al-Qaida or other extremist groups that would threaten the United States. But US officials believe the Taliban maintain ties to al-Qaida.

Austin has asserted that the U.S. military is capable of containing al-Qaida or any other extremist threat to the United States emanating from Afghanistan.

Austin and Secretary of State Antony Blinken appeared together in Qatar on Tuesday in a show of US gratitude for that Gulf state’s help with the transit of tens of thousands of Afghans and others evacuated from Kabul. Blinken also visited an evacuee transit site in Germany, and Austin visited Bahrain and Kuwait.

Together, the Austin and Blinken trips were meant to reassure Gulf allies that President Joe Biden’s decision to end the U.S. war in Afghanistan in order to focus more on other security challenges like China and Russia does not foretell an abandonment of US partners in the Middle East.

Austin had been scheduled to visit Saudi Arabia on Thursday as the final stop on his Gulf tour. But on Wednesday evening his spokesman, John Kirby, announced that the visit had been dropped due to “scheduling issues.” Kirby offered no further explanation but said Austin looked forward to rescheduling.

Austin indicated that his visit was postponed at the Saudis’ request. “The Saudis have some scheduling issues; I can’t speak to exactly what they were,” he said.

The Saudi stop notably was to happen two days before the 20th anniversary of the terrorist attacks on the United States that killed nearly 3,000 people.

U.S. relations with the Saudi government have been strained at times in the intervening years. In 2018, Crown Prince Mohammed bin Salman oversaw an unprecedented crackdown which culminated in the gruesome killing of Washington Post contributing columnist and dissident Jamal Khashoggi by Saudi agents in the Saudi Consulate in Turkey.

Earlier this month, President Joe Biden directed the declassification of certain documents related to the 9/11 attacks, a gesture to victims’ families who have long sought the records in hopes of implicating the Saudi government.

The Saudi government denies any culpability. On Wednesday the Saudi Embassy in Washington released a statement welcoming the move to declassify and release more documents related to 9/11, saying, “no evidence has ever emerged to indicate that the Saudi government or its officials had previous knowledge of the terrorist attack or were in any way involved in its planning or execution.”