WASHINGTON — As it cobbles together additional aid for rebel fighters there, the Obama administration believes Syria could face a protracted, bloody conflict, even if the rebels succeed in ousting President Bashar al-Assad, several officials said Thursday.

The top American intelligence official, James R. Clapper Jr., said that even if Mr. Assad’s government fell, sectarian fighting would most likely engulf the country for a year or more. The American ambassador to Syria, Robert S. Ford, warned that without a negotiated political transition, supporters of the Assad government, “fearing death, would fight to the death.”

Those bleak assessments, delivered in separate hearings before the House and the Senate, underscore the grinding nature of the conflict in Syria and the administration’s pessimism that outside intervention will avert further humanitarian tragedy.

“I agree with you that the prospects in Syria are not good,” said Elizabeth Jones, the acting assistant secretary of state for Near Eastern affairs, who along with Mr. Ford faced sharp questioning from members of the Senate Foreign Relations Committee over what the members said was an inadequate American response to the continuing bloodshed.