WASHINGTON — Secretary of State Antony J. Blinken defended the Biden administration’s handling of its withdrawal from Afghanistan during Senate testimony on Tuesday, even as top intelligence officials separately reported that Al Qaeda could rebuild in the country in a year or two — a shorter timeline than they had estimated before the Taliban seized control.

After more than five hours of House testimony on Monday, Mr. Blinken appeared in person before the Senate Foreign Relations Committee, again conceding no fault as Republicans denounced President Biden’s withdrawal as “the worst foreign policy catastrophe in my lifetime,” as Senator Ted Cruz of Texas put it.

But when pressed about a deadly drone strike in Kabul last month on a vehicle that U.S. officials thought contained an Islamic State bomb, Mr. Blinken conceded that he was unsure of what had happened. New York Times reporting has raised doubts about the military’s version of events, including whether explosives were in the vehicle and whether the driver had a connection to the Islamic State.

“The guy that the Biden administration droned — was he an aid worker or an ISIS-K operative?” asked Senator Rand Paul, Republican of Kentucky, referring to the Afghan branch of the Islamic State.

“I don’t know, because we’re reviewing it,” Mr. Blinken replied. The Pentagon has defended the strike but continues to investigate.

The day amounted to an uncomfortable homecoming for Mr. Blinken, who served as the top Democratic staff member on the panel when Mr. Biden, then a Delaware senator, was its ranking member and chairman some 20 years ago. As on Monday, the secretary of state was courteous and dispassionate, and his mild manner appeared to frustrate some Republican senators.

“OK, I got it,” Senator Ron Johnson, Republican of Wisconsin, said at one point, interrupting one of Mr. Blinken’s typically methodical answers. “It’s bureaucratic speak.”

Mr. Blinken again argued that the Biden administration was strategically wise to withdraw from Afghanistan after a draining 20-year occupation — but also that it was bound by a peace agreement that the Trump administration had struck with the Taliban committing to an exit.

He also insisted that the State Department had planned responsibly for the withdrawal, based on intelligence suggesting that the Afghan government could hold out for months without U.S. military support. The department responded effectively when the government suddenly fell to the Taliban last month, he said.

The secretary of state also noted that he had quadrupled the number of staff members assigned to expedite special immigrant visas for Afghans who worked with U.S. forces in Afghanistan and who might be at risk of Taliban reprisals. He also pointed out that the Trump administration had halted interviews with visa applicants in Kabul in March 2020, after the coronavirus began to spread. “The program was basically in a stall,” Mr. Blinken said.

But committee members said that the Biden administration had failed to adequately plan for the Afghan government’s collapse, and that it should have waited to withdraw until more Americans and at-risk Afghans could be relocated.

Mr. Blinken said that the administration’s worst-case planning began “in the spring and summer,” with interagency meetings coordinated by the White House.

“In the end, we completed one of the biggest airlifts in history, with 124,000 people evacuated to safety,” he said.

Senator Mitt Romney, Republican of Utah, challenged Mr. Blinken’s refrain that the administration was obliged to withdraw rapidly because of the Trump administration’s agreement with the Taliban, which set a May 1 exit date. Mr. Biden ordered all troops to leave by Aug. 31 — after initially announcing a Sept. 11 deadline — and has said that staying longer would have invited attacks by the Taliban, which had agreed under the deal not to attack withdrawing U.S. forces.

Mr. Romney asked Mr. Blinken why U.S. troops could not have stayed past Aug. 31, if the Biden administration had been willing to extend the May 1 deadline.

“Why didn’t you push it much later?” Mr. Romney said, given that “you knew that there was no way you were going to get all these people out in time.”

The military had told the president that it needed three to four months to withdraw from Afghanistan “in a safe and orderly way,” Mr. Blinken replied. He added that the administration had incurred risk of attacks by staying as long as it did.

Senator Marco Rubio, Republican of Florida, accused the administration of setting an arbitrary deadline for political purposes. “We wanted to be out by Sept. 11 so that we could have some ceremony arguing that we’d pulled out of Afghanistan on the anniversary of 9/11,” he said.

The new intelligence assessment does not drastically differ from previous warnings by U.S. officials, including a Pentagon estimate this year that Al Qaeda could reconstitute in Afghanistan in two years. But it reflects revised assumptions given that the Taliban have little ability to control Afghanistan’s borders.

“The current assessment probably conservatively is one to two years for Al Qaeda to build some capability to at least threaten the homeland,” Lt. Gen. Scott D. Berrier, the director of the Defense Intelligence Agency, said at the annual Intelligence and National Security Summit on Tuesday.

The new timeline did not come up at Mr. Blinken’s hearing, but the question of the terrorist threat to the United States from a Taliban-controlled Afghanistan did. When asked by Mr. Rubio about longstanding ties between the Taliban and Al Qaeda, Mr. Blinken conceded that “the relationship has not been severed, and it’s a very open question as to whether their views and the relationship has changed in any kind of definitive way.”

Several Republicans complained about U.S. military equipment supplied to the Afghan security forces that fell into the hands of the Taliban. In images posted online, Taliban fighters have shown off American-made Humvees, helicopters, uniforms, body armor, rifles and night-vision goggles.

Mr. Blinken noted that departing U.S. troops had rendered some of the equipment inoperable, that other items would soon stop working without maintenance, and that the rest presented “no strategic threat to us or any of Afghanistan’s neighbors.”

He also said he would soon appoint a senior State Department official to oversee issues specifically relating to women in Afghanistan, whose rights may be severely curtailed under the Taliban’s radical interpretation of Islam.

In his opening remarks, Mr. Blinken urged the Senate to move faster toward confirming Biden officials, including 80 State Department nominees. He called it “essential that we accelerate the process for national security appointments, since a catastrophic attack could occur with little or no notice.”

But Senator Josh Hawley, Republican of Missouri, said in a floor speech that he would delay key nominees for the Defense and State Departments unless Mr. Blinken, along with Defense Secretary Lloyd J. Austin III and Jake Sullivan, the national security adviser, resign over the chaotic exit from Afghanistan.

While most Republicans portrayed the withdrawal as “an epic failure, with no planning, no strategy,” in the words of Senator John Barrasso of Wyoming, Democrats rose to the administration’s defense.

“It is heartbreaking, what happened,” said Senator Christopher S. Murphy, Democrat of Connecticut. But he added that there was no avoiding a chaotic withdrawal by merely thinking “if we had just had better execution, we could have avoided this panic and confusion.”

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