WASHINGTON — The Pentagon will send almost 4,000 additional American forces to Afghanistan, a Trump administration official said Thursday, hoping to break a stalemate in a war that has now passed to a third U.S. commander in chief. The deployment will be the largest of American manpower under Donald Trump’s young presidency.

The decision by Defense Secretary Jim Mattis could be announced as early as next week, the official said.

It follows Trump’s move to give Mattis the authority to set troop levels and seeks to address assertions by the top U.S. commander in Afghanistan that he doesn’t have enough forces to help Afghanistan’s army against a resurgent Taliban insurgency.

The rising threat posed by Islamic State extremists, evidenced in a rash of deadly attacks in the capital city of Kabul, has only fueled calls for a stronger U.S. presence, as have several recent American combat deaths.

The bulk of the additional troops will train and advise Afghan forces.