WASHINGTON - Unmanned aircraft have proved their usefulness in the war zones of Afghanistan and Iraq. Now pressure's building to allow them in the skies over the United States.

The Federal Aviation Administration has been asked to issue flying rights for a range of pilotless planes to carry out civilian and law enforcement functions but has been hesitant to act.

Officials are worried that they might plow into airliners, cargo planes, and corporate jets that zoom around at high altitudes, or helicopters and hot-air balloons that fly as low as a few hundred feet off the ground.

"They're good for dull, dirty or dangerous jobs, things you wouldn't want to subject a manned plane to," Paul McDuffee, a vice president at Insitu Inc., a Boeing subsidiary that makes drones, told the Pittsburgh Tribune-Review. For example, a drone could be flown into wildfire or storm areas.

They also often cost less than piloted aircraft and can stay aloft far longer.

The FAA said last week it was teaming with Insitu for a two-year study of drones. The agency wants to come up with ways to safely integrate the aircraft into U.S. airspace that commercial airlines use - airspace that is congested in parts such as the Northeast.

The Government Accountability Office also has concerns. "No technology has been identified as a suitable substitute for a person on board the aircraft in seeing and avoiding other aircraft," an agency report said.

Besides, these pilotless aircraft come in a variety of sizes, some the size of small airplanes, others like backpacks.

The risks have not deterred the civilian demand for pilotless planes.

Tornado researchers want to send them into storms to gather data. Energy companies want them to monitor pipelines. State police want to capture images of speeding cars' license plates. Police envision tracking fleeing suspects. Homeland Security wants to expand their use along the borders and coastlines to spot drug smugglers or illegal immigrants.

"There is a tremendous pressure and need to fly unmanned aircraft in [civilian] airspace," Hank Krakowski, the FAA's head of air traffic operations, told European aviation officials recently. "We are having constant conversations and discussions, particularly with the Department of Defense and the Department of Homeland Security, to figure out how we can do this safely with all these different sizes of vehicles."

There are two types of unmanned planes: aircraft programmed to fly a mission and aircraft remotely controlled from the ground, sometimes from thousands of miles away.

"I think industry and some of the operators are frustrated that we're not moving fast enough, but safety is first," Krakowski said in an interview. "This isn't Afghanistan. This isn't Iraq. This is a part of the world that has a lot of light airplanes flying around, a lot of business jets."

Texas officials, including Gov. Rick Perry, Sens. Kay Bailey Hutchison and John Cornyn, and Rep. Henry Cuellar, have been leaning on the FAA to approve requests to use unmanned aircraft along the Texas-Mexico border.

The FAA recently approved one request to use the planes along the border near El Paso, but another request to use them along the Texas Gulf Coast and near Brownsville is still pending.

Cornyn is blocking a Senate confirmation vote on President Obama's nominee for the No. 2 FAA job, Michael Huerta, to keep the pressure on. Other lawmakers want a plan to speed up use of the planes beyond the border.

A bill approved by the Senate gives the FAA a year to come up with a plan; a House version sets the deadline at Sept. 30, 2013, but directs the transportation secretary to give unmanned aircraft permission to fly before the plan is complete, if that can be done safely.

Marion Blakey, a former FAA administrator and president of the Aerospace Industries Association, whose members include unmanned-aircraft developers, said that the agency had been granting approvals on a case-by-case basis but that the pace was picking up.

Some concerns will be alleviated when the FAA moves from a radar-based air traffic control system to one based on GPS technology. Then, every aircraft will be able to continuously advise controllers and other aircraft of its location. However, that's a decade off.

Michael Barr, a University of Southern California aviation safety instructor, said the matter should not be rushed. "All it takes is one catastrophe," Barr said. "They'll investigate, find they didn't do it correctly, there'll be an outcry, and it will set them back years."