But the situation on the ground has shifted rapidly and repeatedly, making this "milestone" a moving target.
Some Americans who requested assistance have not yet departed, and hundreds of others remain in the country who could change their minds and seek a way out, especially because many of those who are staying are doing so only because extended family members who are Afghans have not been able to get out.
"This mission will continue. These numbers are nothing more than a snapshot on any given day. It's not that we're closing up shop, but we are marking an important milestone," the senior State Department official said.
Since Aug. 31, 385 U.S. citizens have departed Afghanistan with U.S. government help, per the State Department, but that number didn't include a flight that departed Thursday for Doha, Qatar.
That number will now grow even more, as the fewer than 80 U.S. citizens still in the country and seeking help will be evacuated in the coming days, according to the senior official. They declined to specify how, but said there will be more chartered flights.
In total, that means nearly 450 U.S. citizens will have soon departed Afghanistan with U.S. government help -- roughly four times as many as Secretary of State Antony Blinken said remained in the immediate aftermath of Biden's Aug. 31 withdrawal.
The agency has previously defended that difference by saying the situation on the ground was constantly shifting.
"The number fluctuates as people change their minds about leaving, or as some U.S. citizens choose to go back, as many have family members in Afghanistan they do not want to leave behind, and we've seen that -- so the number is very fluid," a State Department spokesperson told ABC News Tuesday.
Some lawmakers and advocacy groups have said the number is even higher, with Sen. Ben Sasse, R-Neb., saying last month the administration "has shamelessly and repeatedly lied about the number of Americans trapped behind Taliban lines."
The senior State Department official dismissed some of that "bad-faith" criticism as "tinged with politics and partisanship" and repeated the administration's commitment to giving all U.S. citizens who want out of Afghanistan a way out.
Many Americans who were left behind by the massive evacuation operation in August have also expressed anger and outrage about what they describe as abandonment.
"How can you leave a U.S. citizen with the background that I have, that can be hunted at any time? How can you leave them there?" said Prince Wafa, a 30-year-old naturalized U.S. citizen born in Afghanistan. After serving with U.S. forces for four years and securing a special immigrant visa, Wafa has been living in San Diego, but returned to Afghanistan this summer to help his wife get out.
While Wafa was unable to get a seat on an evacuation flight out before troops left, approximately 6,000 American citizens were evacuated, according to the State Department, out of nearly 124,000 people in total.
The administration still hopes to pick up the pace of flights out of Afghanistan in the coming weeks, especially with help from the Qatari government, which has been arranging chartered Qatari Airways flights. On Friday, Blinken will meet his counterpart, Deputy Prime Minister and Foreign Minister Mohammed bin Abdulrahman Al Thani, for a strategic dialogue where the issue will be among many discussed, the senior official said.
For months now, there have been negotiations among the Qatari and Turkish governments, the Taliban's interim government and private firms about reopening Kabul's international airport. But hope for a swift reopening seems to have faded, in particular because of damage to the airport during the August evacuations and concerns over airport security.
The senior official declined to say how close the parties may be beyond that they were "not there yet" and the agency was "still working closely with our partners" on that goal.
But so far, the Taliban itself has not been an issue, according to the senior official.
"The Taliban have been uneven in some areas, but when it comes to safe passage and allowing those who wish to leave the country to leave, I think they have by and large adhered to that commitment, and I think the milestone we achieved yesterday is a testament to that," the senior State Department official said.
In a joint statement Thursday, delegations from the U.S., Russia, China and Pakistan said they "welcomed the Taliban's continued commitment to allow for the safe passage of all who wish to travel to and from Afghanistan." The diplomats met with senior Taliban leaders on the sidelines of their summit in Islamabad Thursday, according to their statement.
While hundreds of Americans and other foreigners have gotten out, there's been intense criticism about the many Afghans left behind and still seeking departure, especially those who worked for the U.S. military or diplomatic missions and whose lives are now at risk.
"The U.S. military and diplomatic presence in Afghanistan may have ended in August, but the U.S. government's obligation did not," said Sunil Varghese, policy director of the International Refugee Assistance Project, or IRAP, on an advocacy call on Tuesday. "The Biden administration must provide immediate, realistic pathways to safety for these communities."
The senior State Department official declined to say how many Afghan partners the administration has helped evacuate. But they said thanks to the work of nongovernmental partners like veterans groups, a couple thousand have been able to fly out on chartered flights, including some on those arranged by the Qatari government where the U.S. has facilitated seats.
"Even if we reach a point where every American who has raised his or her hand and is ready to leave has departed, our efforts to assist others, that will continue," the senior official added.