WASHINGTON — President Barack Obama and Afghan President Hamid Karzai said Friday they have agreed to speed up slightly the schedule for moving Afghanistan’s security forces into the lead across the country, with U.S. troops shifting fully to a support role.

The leaders also said Obama agreed to place battlefield detainees under the control of the Afghan government.

Obama, appearing in the East Room of the White House with Karzai at his side, said accelerating the transition to Afghan security control this spring would set the stage for further withdrawal of U.S. and other foreign forces, although he did not say how quickly a U.S. drawdown would be carried out this year and next.

There are 66,000 U.S. troops there.

“Starting this spring our troops will have a different mission: training, advising and assisting Afghan forces,” Obama said. “It will be a historic moment.”

He added that even in a backup role he could not rule out that U.S. troops could be drawn into combat. But he emphasized that their main role would be support, such as training and advising.