White House national security adviser Jake Sullivan on Monday acknowledged the security situation in Afghanistan "unfolded at unexpected speed," while maintaining that President Biden stands by his decision to withdraw U.S. troops. 

Heavily armed Taliban fighters swept into Afghanistan’s capital of Kabul on Sunday after the government collapsed, and the Afghani president fled the country, signaling the end of the United States’ 20-year effort to rebuild the nation after the withdrawal of the U.S. military from the region. 

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Sullivan, during an appearance on ABC News’ "Good Morning America" Monday, defended Biden’s decision to withdraw troops.

"The president did not think it was inevitable that the Taliban were going to take control of Afghanistan," Sullivan said. "He thought the Afghan national security forces could step up and fight because we spent 20 years, tens of billions of dollars, training them, giving them the best equipment, giving them support of U.S. forces for 20 years." 

"When push came to shove, they decided not to step up and fight for their country," Sullivan said, adding that the president was faced with the question of whether U.S. men and women should be "put in the middle of another country’s civil war when their own army won’t fight to defend them?" 

"And his answer to the question was ‘no,’ and that is why he stands by this decision," Sullivan said. 

The Taliban is pushing to restore the Islamic Emirate of Afghanistan, the formal name of the country under the Taliban rule before the militants were ousted by U.S-led forces in the wake of the 9/11 attacks, which were orchestrated by Al Qaeda while it was being sheltered by the Taliban. 

Taliban fighters stand guard in front of the Hamid Karzai International Airport, in Kabul, Afghanistan, Monday, Aug. 16, 2021. Thousands of people packed into the Afghan capital's airport on Monday, rushing the tarmac and pushing onto planes in desperate attempts to flee the country after the Taliban overthrew the Western-backed government. (AP Photo/Rahmat Gul) (AP Photo/Rahmat Gul)

Last week, though, as the Taliban seized major provinces throughout the country, the Biden administration assessed that Kabul could fall to the Taliban within 90 days. A prior CIA assessment months ago said Kabul could fall in six months, however, officials last week said that prediction was cut in half. 

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The White House, at the time, said the Afghan National Defense and Security Forces had "what they need" to "fight back." 

Sullivan, Monday morning, admitted that the security situation in Kabul escalated at a much faster pace than the administration anticipated. 

"We planned for a wide range of contingencies, one of those was the need to flow in a substantial number of forces to secure the airport and be able to facilitate that evacuation," Sullivan explained. "We have now successfully drawn down the U.S. embassy in Kabul. We have moved all of those personnel either to the airport or out of the country. And we flowed in several thousand troops who had been prepositioned in theater for precisely this purpose." 

Sullivan said the president "activated that deployment well before the fall of Kabul, because as we watch this situation unfold, and it certainly unfolded at unexpected speed, we put that contingency plan in place." 

Sullivan said the administration plans to continue the evacuation of American citizens from Afghanistan, including those who work for the U.S., like interpreters and translators and "other vulnerable Afghans at risk." 

"We are working to do that, first, by securing the airport today and then in the days ahead, by taking people out one flight a time, flight after flight," Sullivan said. "We fully intend to continue an evacuation process to bring out people who worked alongside of us in Afghanistan." 

At the Kabul International Airport Monday, U.S. forces attempted to gain control of the crowd of Afghans desperate to leave the country. As of Monday morning, all U.S. evacuation flights were suspended, with U.S. officials telling Fox News that the runway was "not secure" as hundreds of Afghans "breached" the airport walls and flooded the runway. 

Officials said until the runway is cleared, the U.S. military flights are suspended. The U.S. closed its embassy in Kabul on Sunday. 

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As for the Taliban, Sullivan warned that the Biden administration is "going to hold the Taliban accountable to not allowing Al Qaeda to have a safe haven there." 

"If they do, there will be costs and consequences for the Taliban in addition to the direct suppression of Al Qaeda that we will have the capability to pursue in the period ahead," Sullivan said. 

As for the American people, Sullivan said they can expect to hear from the president "soon," noting that he is "actively engaged with his national security team" and "working the situation hard." 

"He is focused on ensuring that the mission, which is to secure that airport and to continue these evacuations, that the mission continues and is brought to a positive conclusion." 

He added: "That is his overriding focus right now. He’s deeply engaged in it, and yes, at the right point, he will absolutely address the American people." 

Sullivan went on to explain that the United States went to Afghanistan for "one reason," and that reason was to "get the people who attacked us on 9/11." 

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"A decade ago, we got Usama bin Laden, we degraded Al Qaeda, we stopped terrorist attacks against the United States from Afghanistan for 20 years," Sullivan said. "What the president was not prepared to do was enter a third decade of conflict flowing in thousands more troops, which was his only other choice to fight in the middle of a civil war that the Afghan army wouldn’t fight for itself." 

He added: "He would not do that to America’s men and women or their families. And that is why he made the decision to withdraw U.S. forces from Afghanistan this year." 

Fox News' Lucas Tomlinson and the Associated Press contributed to this report.