The United States has deployed Predator drones to hunt for al-Qaeda operatives in Yemen for the first time in years but has not fired missiles from the unmanned aircraft because it lacks solid intelligence on the insurgents' whereabouts, senior U.S. officials said.

The use of the drones is part of a campaign against an al-Qaeda branch that has claimed responsibility for near-miss attacks on U.S. targets that could have had catastrophic results, including the recent plot to place parcels packed with explosives on cargo planes.

U.S. officials said the Predators have been patrolling the skies over Yemen for several months in search of leaders and operatives of the group al-Qaeda in the Arabian Peninsula, or AQAP. After withstanding a flurry of attacks involving Yemeni forces and U.S. cruise missiles earlier this year, AQAP's leaders "went to ground," a senior Obama administration official said.

The use of U.S. drones in Yemen underscores the deep U.S. reliance on what has become a signature weapon against al-Qaeda and other terrorist groups.

The deployment also represents an attempt by the Obama administration to reinvigorate a campaign that has gone without a visible U.S. strike for nearly six months. Officials praised Yemeni cooperation and said they have been given wide latitude. Pressed on whether the drones would be free to shoot, a second administration official said, "The only thing that does fall into the 'no' category right now is boots on the ground."

The officials and others spoke on the condition of anonymity to discuss sensitive military and intelligence operations.

Yemeni officials said the United States had not yet pushed for the use of Predator-fired missiles and indicated that they had deep reservations about weapons they said could prove counterproductive.

"Why gain enemies right now?" said Mohammed A. Abdulahoum, a senior Yemeni official. "Americans are not rejected in Yemen; the West is respected. Why waste all this for one or two strikes when you don't know who you're striking?"

Instead, Yemen has asked the administration to speed up shipment of promised helicopters and other equipment for its own use, and to recognize the backlash that a more visible U.S. campaign could cause. A U.S. defense official said plans were being made to nearly double military aid, to $250 million, in 2011.

Senior administration officials said that cooperation with Yemeni President Ali Abdullah Saleh has intensified in the aftermath of the parcel bomb plot and that the subsequent shutdown of commercial and cargo flights from Yemen focused the government's attention on the cost of AQAP's presence in the country. Officials said Saleh had been pushed in extensive talks last week to expand Yemen's own effort and allow increased U.S. action.

"Where we are right now with our capabilities, with our platforms, and with our authorities and permissions," the U.S.-Yemeni pursuit of al-Qaeda "might look very different in 12 months or 18 months," the senior Obama administration official said.

U.S. officials described a major buildup of intelligence and lethal assets already underway, including the arrival of additional CIA teams and up to 100 Special Operations force trainers, and the deployment of sophisticated surveillance and electronic eavesdropping systems operated by spy services including the National Security Agency.