A US Defense Department spokesperson says some American troops in Afghanistan have begun "retrograde", or withdrawal from Kabul airport.
Pentagon Press Secretary John Kirby said this at a news briefing on Saturday, as the August 31 deadline for the US military to withdraw from Afghanistan approaches.
The State Department says the number of US citizens still looking to leave Afghanistan has dropped to about 350.
The White House said the US evacuated roughly 6,800 people from Kabul in the 24-hour period through early Saturday.
That is about half the figure for the previous day, and includes Afghan translators and other local allies.
Vigilance for terror attacks has been heightened in the vicinity of Kabul's airport, following Thursday's suicide bombing that killed many civilians and 13 US service members.
The US Embassy in Afghanistan issued a security alert on Sunday. It called on US citizens to leave the area around the airport immediately, citing a "specific, credible threat."
President Joe Biden also said on Saturday that another attack is highly likely in the next 24 to 36 hours.