Updated

The White House has not completely shut the door on leaving U.S. troops in Afghanistan beyond the Aug. 31 deadline.

"During a meeting this morning with the G7 leaders, the President conveyed that our mission in Kabul will end based on the achievement of our objectives," White House Press Secretary Jen Psaki said in a statement Tuesday.

Psaki added that the current evacuation effort is "on pace to finish by August 31st," but also warned that troops on the ground face "increasing threats from ISIS-K" as the mission evolves.

U.S. TROOPS WILL HAVE TO BEGIN AFGHANISTAN PULLOUT ON FRIDAY TO MEET DEADLINE

"[C]ompletion of the mission by August 31st depends on continued coordination with the Taliban, including continued access for evacuees to the airport," Psaki said. "In addition, the President has asked the Pentagon and the State Department for contingency plans to adjust the timeline should that become necessary."

Psaki did not clarify when exactly the August 31 deadline would go into effect. In a news conference on Tuesday, a reporter asked her whether the deadline would go into effect at midnight on August 31 U.S. Eastern time or Afghanistan time. "That's really a great question," Psaki said. She pledged to "get back to you on that."

Psaki also confirmed that the evacuations will stop before August 31 in order to allow the military to withdraw to meet the deadline.

Fox News was the first to report Tuesday that some U.S. troops have already started to leave Afghanistan. A U.S. official stressed, "We can still defend the airport," while adding forces still on the ground had the capability to "get Americans out who want to evacuate."

Pentagon Press Secretary John Kirby said Tuesday evening, "commanders on the ground are empowered to make any adjustments they see fit, when they see fit. That includes changes to the footprint. To that end, we can confirm reports of the departure from Afghanistan of several hundred U.S. troops. These troops represent a mix of headquarters staff, maintenance and other enabling functions that were scheduled to leave and whose mission at the airport was complete. Their departure represents prudent and efficient force management.  It will have no impact on the mission at hand."

Earlier Tuesday, Kirby said that the White House is still "aiming toward the end of the month" to completely draw down U.S. troops in Afghanistan.

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"We continue to make progress every day in getting Americans – as well as Special Immigrant Visa applicants and vulnerable Afghans – out," he said. "We remain committed to getting any and all Americans that want to leave, to get them out. We still believe – certainly now that we have been able to increase the capacity and the flow – we believe that we have the ability to get that done by the end of the month." 

The Pentagon added that the number of U.S. troops in the country will go to "zero" by the agreed upon deadline.

Fox News' Tyler O'Neil and Lucas Y. Tomlinson contributed to this report.