The U.S. will keep at least 5,000 military personnel in Afghanistan after 2016, according to a congressional aide, slowing the administration's withdrawal timetable and likely ensuring that America's longest war will endure beyond President Barack Obama's term in office.

The change is set to be announced by the White House on Thursday, according to the aide, who asked for anonymity to discuss a private briefing. It marks the second time Obama has had to alter his schedule to wind down U.S. military operations following persistent Taliban attacks and setbacks in building up Afghan government forces.

The almost 10,000 U.S. military personnel now in Afghanistan were scheduled to be drawn down through 2016, with all but 1,000 troops pulled out by early 2017. Many Afghans have said that they won't be ready to carry the anti-Taliban fight alone by the end of next year.