WASHINGTON  Obama administration officials tried again on Thursday to reassure members of Congress anxious about the military buildup in Afghanistan, telling them repeatedly that American troops can begin to withdraw in July 2011. But the lawmakers seemed more interested in how long the withdrawal would take.

Defense Secretary Robert M. Gates said July 2011 “will be the beginning of a process, an inflection point, if you will, of transition for Afghan forces as they begin to assume greater responsibility for security.” He went on to say that the pace of the withdrawal would be determined by “conditions on the ground.”

“It will be a gradual but inexorable process,” Mr. Gates told the Senate Foreign Relations Committee, in remarks on President Obama’s plan to ship about 30,000 additional troops to Afghanistan next year, bringing the total American troops there to about 100,000.

Several members of the Foreign Relations Committee clearly wanted far more specifics than Mr. Gates offered in his nuanced remarks. Indeed, Senator Robert Menendez, Democrat of New Jersey, was skeptical even about the July 2011 target date for beginning to bring home the troops, calling it “clearly aspirational.”