Updated

The situation in Syria is too complicated right now to provide opposition forces with lethal aid, a senior U.S. military commander said during congressional testimony Tuesday.

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

But Mattis also told the Senate Armed Services Committee that Assad's support is eroding daily. "He is losing ground," Mattis said of Assad.

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.

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

As proof the Afghans are taking the lead on most operations, Mattis said U.S. forces have suffered only a small number of casualties since the beginning of 2013 while nearly 200 Afghan troops have been killed during the same period.

President Barack Obama announced last month that he will cut the size of the U.S. force in Afghanistan roughly in half by a year from now. There are currently about 66,000 U.S. troops there, and he said he will withdraw about 34,000 by this time next year.

Mattis said he backs the president's plan. "I support the pace and I support the numbers," Mattis said.

Headquartered at MacDill Air Force Base in Tampa, Fla., Central Command is responsible for operations in a swath of the globe that reaches from Central Asia to the Horn of Africa, a region where religious extremism has driven al-Qaida and other terrorist groups.