WASHINGTON — The Biden administration is preparing for Afghanistan’s capital to fall far sooner than feared only weeks ago, as a rapid disintegration of security has prompted the revision of an already stark intelligence assessment predicting Kabul could be overrun within six to 12 months, according to current and former U.S. officials familiar with the matter.

One official, who like others spoke on the condition of anonymity due to the issue’s sensitivity, said Tuesday that the U.S. military now assesses a collapse could occur within 90 days. Others said it could happen within a month. Some officials said that although they were not authorized to discuss the assessment, they see the situation in Afghanistan as more dire than it was in June, when intelligence officials assessed a fall could come as soon as six months after the withdrawal of the U.S. military.

“Everything is moving in the wrong direction,” said one person familiar with the military’s new intelligence assessment.

The worsening outlook comes as Taliban fighters, emboldened by the American military departure, have steadily retaken ground from Afghan government forces - including at least seven provincial capitals in a span of days. Nevertheless, President Joe Biden on Tuesday insisted that his decision to withdraw U.S. forces is not up for debate, saying that despite the Afghans’ weak performance militarily, he did not “regret” his decision to end the 20-year campaign and he is not considering any change of plans in light of the Taliban’s gains.

“Look,” Biden told reporters at the White House, “we spent over a trillion dollars over 20 years. We trained and equipped, with modern equipment, over 300,000 Afghan forces. And Afghan leaders have to come together.”

As security in Afghanistan continues to erode, conversations within the U.S. government have turned to the U.S. Embassy in Kabul and whether Washington should keep it open - and for how long. For now, the State Department has said its posture is unchanged.

“Obviously it is a challenging security environment,” State Department spokesman Ned Price acknowledged Tuesday. “We are evaluating the threat environment on a daily basis.”

A Pentagon official, however, said military planners have been working under the assumption for some time that the evacuation of American diplomats and other nonmilitary personnel from Afghanistan could be necessary on short notice, and that some scenarios envision the fall of Kabul within 30 to 90 days.

Separately, John Kirby, the Pentagon’s chief spokesman, said Tuesday that as a matter of policy, the Defense Department does not discuss intelligence assessments.

“We are mindful of deteriorating security conditions in parts of the country, but no particular outcome is inevitable,” Kirby said. “We will continue to coordinate airstrikes with - and in support of - Afghan forces when and where feasible. But as the president made clear, Afghan leaders have to come together.”

With the American military departure set to be formalized Aug. 31, U.S. support for the Afghans has waned considerably and now is conducted from afar. Unless Biden changes policy, the limited U.S. airstrikes now being employed against the Taliban are expected to end this month, with the completion of the military withdrawal. Kirby declined to speculate whether that policy could change.

White House press secretary Jen Psaki said the onus is now on Afghan leaders.

“The president continues to believe that it is not inevitable that the Taliban takes over Kabul or the country, and that they need to show political will at this point to push back, and obviously there’s a political process that we continue to support,” Psaki said, referring to long-languishing negotiations between the Taliban and the Afghan government that have taken a back seat amid the Taliban offensive.

Biden made his decision to pull out U.S. forces with knowledge of the potential consequences, she added.

“He asked for a clear assessment for review from his team on what the possible implications could be,” she said. “He asked them not to sugarcoat that, he asked them to lay out specifically and clearly what the consequences could be.”

Price, the State Department spokesman, said the Afghan national security forces “far outnumber the Taliban,” with a “capable fighting force of 300,000 troops.” U.S. assessments in the past have indicated there are fewer than that due to corruption in the Afghan military that includes “ghost soldiers” - personnel accounted for on paper but who don’t show up to do their jobs. Other soldiers have fled their posts in recent days when faced with threats by the Taliban.

Price and Kirby also pointed out that the United States has provided the Afghan military with modern weaponry that includes an air force. But the Taliban has been seen in recent days using an array of weapons and equipment that it seized from the Afghan government, including vehicles that still have Afghan army insignia on them.

After weeks of encircling provincial capitals, the Taliban seized their first on Friday, taking control of Zaranj in western Nimruz province. The militants seized three more on Sunday, including the strategically important northern city of Kunduz.

Fighting is fierce and ongoing in several other capitals, including the southern cities of Lashkar Gah and Kandahar, where the Afghan government has deployed its elite commandos. A U.S. official said it is predicted that at least nine provincial capitals could be under Taliban control soon.

The Biden administration has declined to provide details about where the U.S. military has carried out airstrikes in recent days, but Afghan officials have cited their involvement in bombings in Kapisa and Helmand provinces. The Afghan air force has carried out many more strikes and in more locations, U.S. officials said.

Bruce Riedel, a former CIA officer who led a review of U.S. policy toward Afghanistan and Pakistan for President Barack Obama in 2009, said the situation in Afghanistan “is bleak, worse than most expected this quickly.”

“The danger,” he added, “is that the momentum of the Taliban’s offensive will overwhelm the Afghan government and the defense of Kabul will collapse.”

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The Washington Post’s Missy Ryan and Alex Horton contributed to this report.