The Biden administration is making a final effort before May 1 to show progress in slow-moving negotiations between the Taliban and the Afghan government in Doha, Qatar. The Taliban, according to American officials, are stalling.

The administration is pushing the two sides to participate in a peace conference in Turkey to demonstrate progress. Simultaneously, the American and Taliban negotiators continue to try to cement a 90-day reduction in violence, but so far, both sides have hesitated to agree.

The classified intelligence assessment of the Taliban largely taking control assumes that the Afghan government and the Taliban fail to reach a political agreement and that a civil war would erupt after the American exit.

Administration officials warned that making any intelligence estimate is challenging, that predictions about the future are always imprecise and that various factors influence the analysis.

For example, intelligence estimates depend on whether international funding for the Afghan government remains in place. The more money the United States and its allies provide Afghanistan, the longer the government in Kabul should be able to retain control of some of the country. But some officials said that history shows that once American troops are withdrawn, Congress moves quickly to cut financial support for partner forces.

There is also a debate in Washington about the seriousness of the threat of a return of terrorist groups. For now, the number of Qaeda and Islamic State militants in Afghanistan is very small, a senior U.S. official said.

Some senior lawmakers with access to the classified assessments said that it was not certain that if the United States withdrew that Al Qaeda could rebuild a base in Afghanistan from which to carry out terrorist attacks against the United States.

“What is that threat really going to be?” Representative Adam Smith, Democrat of Washington State and the chairman of the House Armed Services Committee, said this week during a virtual conference on Afghanistan. “This isn’t the 1990s when Al Qaeda set up camps, and they had the Taliban and no one was paying attention to them.”

Mr. Smith said keeping U.S. troops in Afghanistan actually increased the risk to Americans there, incurred greater financial costs and handed a propaganda victory and recruiting tool to the United States’ enemies.

Some counterterrorism officials believe that Al Qaeda would prefer to re-establish its headquarters in Afghanistan, should American troops withdraw. But other officials said Al Qaeda’s leadership might be just as likely to look to Africa or the Middle East.

While American intelligence officials have been mostly focused on the threat of Al Qaeda, senior military officials have also raised the prospect of a growth in the power of the Afghanistan arm of the Islamic State.

But in recent years, the Taliban have been at odds with the Islamic State. The two groups have fought, and the Taliban have for the most part pushed back Islamic State forces.

“I can’t imagine a scenario where ISIS and the Taliban would strategically cooperate or collaborate in Afghanistan,” said Lisa Maddox, a former C.I.A. analyst. “The Taliban is an ideological organization, and that ideology is Afghan-centric and not aligned with ISIS’ overarching goals.”

The intelligence estimate predicted that the Taliban would relatively swiftly expand their control over Afghanistan, suggesting that the Afghan security forces remain fragile despite years of training by the American military and billions of dollars in U.S. funding.

Offensives last year in Kandahar and Helmand Provinces, two areas in the country’s south where the Taliban have long held sway, demonstrated that the police and local forces are unable to hold ground, prompting elite commando forces and regular army troops to take their place — a tactic that is likely unsustainable in the long run.

The Afghan security forces still rely heavily on U.S. air support to hold territory, which American military leaders acknowledged this week. It is unclear whether that American air power would continue if U.S. forces left Afghanistan, perhaps launched from bases in the Persian Gulf, although the Pentagon has drawn up such options for the White House.

“The capabilities that the U.S. provides for the Afghans to be able to combat the Taliban and other threats that reside in Afghanistan are critical to their success,” Gen. Richard D. Clarke, the head of Special Operations Command, told the Senate on Thursday.

Julian E. Barnes and Eric Schmitt reported from Washington, and Thomas Gibbons-Neff from Kabul, Afghanistan. Najim Rahim contributed reporting from Kabul.

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