The U.S. should keep troops in Afghanistan after the combat mission ends next year, but a decision on the number can wait until the Afghan army has been tested in this summer’s fighting, the top U.S. commander in Kabul said Tuesday.

In his first congressional testimony since taking over in February, Marine Gen. Joseph Dunford told the Senate Armed Services Committee that the U.S. should make an assessment in November, at the customary end of the fighting season.

“My strongest military advice is not to pin down a number right now,” Dunford said under questioning by a skeptical Sen. John McCain, R-Ariz. Before Dunford could finish his thought, McCain interjected, “Don’t you understand, general, that one of the reasons why we’re having so much difficulty in some areas is because the Afghans don’t know what our commitment is?”