WASHINGTON (AP) -- A senior U.S. military commander says the situation in Syria is too complicated right now to provide opposition forces with lethal aid.

Marine Corps Gen. James Mattis, the top officer at U.S. Central Command, said Tuesday he is concerned that U.S. enemies might wind up with weapons that are given to the rebels fighting Syrian President Bashar Assad.

Assad's support is eroding daily, Mattis said during testimony before the Senate Armed Services Committee.

Secretary of State John Kerry announced last week that the U.S. would for the first time provide rebel fighters in the Free Syrian Army with non-lethal assistance -- rations and medical assistance.

Mattis also said the U.S. mission in Afghanistan is succeeding because the Afghan military and security forces are doing most of the fighting.