President Donald Trump has given the Pentagon unilateral authority to send thousands of new American troops to Afghanistan at its discretion, a U.S. official said Tuesday, clearing the way for the U.S. military to intensify its fight against the Taliban and Islamic State extremists in the region.

The decision to delegate control over troop decisions to the Pentagon, which is expected to be announced Wednesday, sets the stage for U.S. commanders to begin sending more forces to the country, after years of reductions made in the hope Kabul could handle internal threats on its own.

Earlier Tuesday, Defense Secretary Jim Mattis told the Senate Armed Services Committee that the U.S. is “not winning” the war in Afghanistan and that he was ready to take steps to address resurgent Taliban militants.

The White House decision to cede authority to Mr. Mattis is another reflection of Mr. Trump’s push to give the military wide latitude around the world. The White House has already given the Pentagon more power to carry out strikes in Yemen and Somalia.