WASHINGTON—President Barack Obama is close to authorizing a mission led by the U.S. military to train moderate Syrian rebels to fight the regime and al Qaeda-linked groups, a move that would significantly expand Washington's role in the conflict, U.S. officials said.

A new military training program, if implemented, would supplement a small train-and-equip program led by the Central Intelligence Agency that Mr. Obama authorized a year ago. U.S. officials don't discuss the CIA's limited training program because it is covert.

In a commencement address at the United States Military Academy at West Point on Wednesday, officials said Mr. Obama will signal backing for the new training effort by saying he intends to increase support to the armed opposition to fight the regime of Syrian President Bashar al-Assad, including by providing them with training.

Mr. Obama isn't expected to provide details about how, or where, that training would be done.

"The president will make clear his intention to expand our support to the moderate Syrian operation and increase our support to Syria's neighbors, who are dealing with the terrorist threats emanating from the situation Assad has created in Syria," a senior administration official said.