WASHINGTON — The top American commander in Afghanistan faces skeptical lawmakers amid concerns that worsening security conditions demand a greater number of U.S. forces to ensure the gains made in the war-torn country since 2001 aren’t lost.

Army Gen. John Campbell is slated to testify today before the House Armed Services Committee, where members are expected to press him on President Barack Obama’s plan to cut American troop levels from 9,800 to 5,500 before he leaves office next January. Obama had backtracked from his initial plan to reduce the U.S. force to 1,000 by the end of 2016.

Republicans have long assailed Obama’s exit strategy, arguing that conditions on the ground in Afghanistan, not a calendar, should determine the pace of the withdrawal. With the Taliban staging new offensives and the Islamic State extremist group seeking a presence in Afghanistan, congressional Democrats also are raising the prospect of an extended stay.

“I’ve always believed that putting a time limit on it is a mistake,” Rep. Mac Thornberry, R-Texas, the committee chairman, said Monday. “To say this is going to take five years, 10 years or 50 years, nobody can say that.”

Rep. Seth Moulton, D-Mass., warned against a repeat of Iraq. American forces were withdrawn too rapidly and without a long-term political strategy to ensure the progress they made would hold, he said. U.S. troops had to return to Iraq after the resulting instability allowed IS to grow.

The mission of the U.S. troops in Afghanistan is to conduct counterterrorism operations and to train and assist the Afghan security forces. The U.S. has committed $113 billion since 2002 for reconstruction projects in hopes of establishing a stable, functioning Afghan government.