TOKYO - The U.S.-made RQ-4 Global Hawk spy plane looks like an upside-down double-decker bus with wings, flies slowly, offers zero leg room - and is one of the most coveted pieces of military technology in the world.

Impressed by its successes in combat for the U.S. Air Force in Iraq and Afghanistan, countries around the globe are lining up to buy the unmanned "drone" aircraft to bolster their own intelligence abilities, despite concerns that exports might send sensitive technology into the wrong hands.

Some experts said the export of the planes could also heighten tensions with countries like China, Iran and Russia - who could be the subject of closer observation and perceive the drones' operations as offensive threats.

Undaunted by such concerns, Northrop Grumman, the producer of the Global Hawk, just wrapped up an Asian tour in Tokyo with a full-sized mock-up and says that along with Japan other countries considering adding the plane to their air forces are South Korea, Australia, Singapore, Britain, Spain, New Zealand and Canada.

Germany has already contracted for a variant called the EuroHawk to be delivered this year.

"That you're going to see a lot more of these airplanes is the bottom line," said Curtis Orchard, vice president Japan for Northrop Grumman Corp. Aerospace Systems. "There is still a 'boutiquey' feeling now, but there is going to be widespread usage."

Along with the armed Predator, the Global Hawk, which does not carry weapons, is one of the most successful of the new generation of drones, or Unmanned Aerial Systems, and has become a staple of Air Force operations in Afghanistan and Iraq.

The aircraft is increasingly being used for civilian responses to natural disasters or for scientific research. It recently flew disaster relief missions in support of the Haiti earthquake, providing thousands of images to recovery and relief agencies.

But military operations are where the plane has made its biggest mark.

Since its introduction 11 years ago, the Global Hawk has put in 40,000 flight hours - 75 percent of that in combat. It can fly at altitudes of 60,000 feet for more than 32 hours at a time, meaning that it can carry out a wide range of intelligence, surveillance and reconnaissance missions above most countries' defenses that would not be possible with manned aircraft.

"The Air Force just can't get enough of the Global Hawk," said Gemma Loochkartt, the Global Hawk Communications representative for Northrop Grumman Aerospace Systems, in San Diego, California.

She said the Global Hawk fleet is now 21 planes finished or under construction, and that was likely to grow substantially.

Spy capabilities and the relatively low cost - $5 million for a Predator and $30 million for a Global Hawk, compared to $85 million for the manned Joint Strike Fighter, which is still in development - have made top-of-the-line drones an attractive option for U.S. allies.

Japanese defense officials, for example, have said they are looking into introducing the plane in their next 5-year military plan as a means of watching neighboring North Korea, which is well within the Global Hawk's 10,000-nautical mile range.

South Korea is believed to be interested in acquiring them for similar use.

But Northrop Grumman's sales effort is a delicate one - the Global Hawk is the most sophisticated spy drone in the Air Force, and not all want to share the eye-in-the sky edge the U.S. now enjoys. The political stakes are also high - North Korea, China, Russia and other countries are not likely to welcome spy planes loitering off their coast.

"I think there is a concern, not just in Asia but wherever the spy planes might be sold, of increased tensions if exports are given the go-ahead," said Takehiko Yamamoto, a professor of international relations at Tokyo's Waseda University. "China, Russia and others would perceive it as a potentially offensive capability. For Japan, I think it is a bad idea."

Drone technology is tightly regulated, as is also the case with stealth fighters such as the F-22, which also has brought interest from Japan and other potential buyers around the world but has been red-lighted for export by Congress.

Sales of the drones are controlled under the Missile Technology Control Regime, a pact among 34 countries that is meant to limit the spread of missile technology and the proliferation of weapons of mass destruction.

Defense Secretary Robert Gates told the Senate Appropriations Committee in March that he shared the fears that drone technology could get into the wrong hands, citing in particular terrorist groups. But he also noted that potential adversaries - particularly Iran - are already developing their own drone fleets, and said sharing technology with allies would be in Washington's best interest.

Orchard acknowledged that the international restrictions remain a major hurdle in building an international market for drones.

"It is a significant issue," Orchard said. "But the Global Hawk is not armed, so that makes it a little easier."

Analyst Lance Gatling, of the Tokyo-based Nexil Research, said exports to close U.S. allies are almost inevitable.

"Someone is always going to complain when someone else buys a system like the Global Hawk," he said. "There certainly are sensitive technologies. It definitely has an impact on any country's ability to observe someone else."

But, he added, "it's a capability the U.S. wants them to have."