President Biden and his national security team are "taking responsibility" for "every decision" the U.S. government took with respect to Afghanistan, White House national security adviser Jake Sullivan said Tuesday, while maintaining that the administration will remain "persistently vigilant" against the terrorist threat to the U.S. homeland. 

Sullivan, during a briefing with reporters at the White House Tuesday, was asked whether the president took responsibility for the "chaos" in Afghanistan after the fall of the country to the Taliban. He called the scenes at Kabul's airport Monday "heartbreaking."

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"The president is taking responsibility for every decision the U.S. government took with respect to Afghanistan, because, as he said, the buck stops with him," Sullivan said, adding that he, as national security adviser, is "also taking responsibility." 

"And so are my colleagues – the secretary of state, the secretary of defense, the chairman of the joint chiefs of staff, directors of the intelligence agencies," Sullivan said. "We, as a national security team, collectively take responsibility for every decision – good decisions – every decision that doesn't produce perfect outcomes." 

He added: "That is what responsibility is." 

Sullivan, echoing the president's remarks Monday, said that while the administration is taking responsibility, that "doesn't change the fact that there are other parties here responsible as well who have taken actions and decisions that helped lead us to where we are." 

"From our perspective, what we have to do now, is focus on the task at hand, the mission at hand," Sullivan continued. "At the end of the day, the question is, can we effectively evacuate those people who we intend to evacuate, and that is what we are planning for, beginning today — airport is secure, flights are going, people are coming." 

"We will take that day by day," Sullivan said, adding that administration officials are staying "in close touch with allies and partners." 

Sullivan, though, admitted that the worsening security crisis is presenting "broader challenges posed by the new realities in Afghanistan." 

"We will remain persistently vigilant against the terrorism threat in Afghanistan," Sullivan said, adding that the U.S. "has proven in other places that we can suppress terrorism without a permanent military presence on the ground." 

"We are going to have to deal with the potential threat of terrorism from Afghanistan going forward, just as we have to deal with potential threat of terrorism in dozens of countries, in multiple continents around the world," Sullivan said, pointing to the terrorist threats from Yemen, Somalia, Syria, "across the Islamic Maghreb," as well as Al Qaeda and ISIS-K. 

Sullivan said the U.S. is prepared to do so by using a "wide variety of tools, intelligence capabilities, defense capabilities, and yes, in some cases, support we can provide to local partners to help them deal with the challenge." 

Sullivan, though, said that the U.S. has "been successful to date in suppressing the terrorist threat to the U.S. homeland in those countries, without sustaining a permanent military presence, or fighting a war." 

"That is what we intend to do with respect to Afghanistan as well," he said. 

"So the question is not whether we are clear-eyed about the terrorist challenge from Afghanistan. It is about whether the terrorist challenge in 2021 is fundamentally different from the terrorist challenge in 2001," Sullivan continued. 

"And we believe it is fundamentally different, and we need to be postured effectively to deal with the terrorism challenges we find today, as opposed to 20 years ago," he added. 

As for the current threat to the U.S. mission, the Pentagon said Tuesday that U.S. commanders are interacting with the Taliban in Kabul, and stressed that, at this point, there has been "no threat" made by the extremist group. 

However, Sullivan admitted that much of the billions of dollars of weaponry the U.S. has given to Afghanistan has been taken by the Taliban. 

"We don't have a complete picture of where every article of defense materials have gone, but certainly, a fair amount of it has fallen into the hands of the Taliban," Sullivan told reporters Tuesday. 

"Obviously, we don't have a sense that they are going to readily hand it over to us at the airport," he added. 

Meanwhile, a White House official told Fox News Tuesday that military and civilian flights from Hamid Karzai International Airport in Kabul have resumed, and that over the last 24 hours the U.S. has been able to move more than 700 people out of Afghanistan, including 150 American citizens. 

Flights were temporarily suspended from Kabul's airport on Monday, as U.S. forces attempted to gain control of the crowd of Afghans desperate to leave the country. 

Despite the Pentagon's assurances Tuesday that there has been "no threat" by the Taliban to the U.S. mission, former senior defense officials in contact with commanders on the ground in Kabul have told Fox News that the Taliban "has a ring outside of the airport and won't let anyone inside it." 

And an Afghan former State Department contractor told Fox News that Taliban fighters have established checkpoints throughout the city and around the airport. Some, he said, are beating people on the way to the airport. 

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Sources have also told Fox News that the Taliban are going through neighborhoods looking for those who worked with the U.S. government. 

President Biden and the Pentagon, though, have warned the Taliban that any threat to the U.S. mission will be met with a swift response. 

"Any attack on our people, or on our operations at the airport, will be met with a swift and forceful, an unambiguous response," Pentagon press secretary John Kirby said during an interview with CNN Tuesday. "And I think I'm just gone leave it at that."

Fox News' Jennifer Griffin, Rich Edson and Bret Baier contributed to this report.