WASHINGTON (Reuters) - Prowling the skies of Pakistan and Yemen, armed drones are America's weapon of choice in its war against al Qaeda, but they are unlikely to play a major role in any U.S. strike against Syria, underscoring the limitations of unmanned aircraft.
Drones do not have the capability for air-to-air combat and would be vulnerable to Syria's defense system of surface-to-air missiles and radar which can track and shoot down warplanes, never mind slower-moving drones.
The Hellfire missiles generally carried by drones also lack the firepower of a cruise missile, which is considered the likely weapon for any limited U.S. strike against President Bashar al-Assad's forces. Washington blames Assad's government for a chemical weapons attack near Damascus last month.
"It's well-known that the Syrian air defense system is robust," a U.S. defense official said. "Drones, like any other (aerial) platform, are vulnerable to integrated air defenses."
Used for protecting American troops in largely uncontested air space in Iraq and Afghanistan, and killing terrorism suspects in Pakistan and Yemen, drones can be remotely piloted from bases in the United States, avoiding risk to the lives of U.S. military personnel operating them.
U.S. drone strikes in Pakistan and Yemen increased dramatically under President Barack Obama and the pilotless aerial vehicles have become a key part of the fight against al Qaeda. The United States has also used them over Afghanistan, Somalia, Libya and Iraq, and this year received approval to base drones in Niger.
However, the situation in Syria is not suited to the use of armed drones - not at least for the moment.
"If we don't control the air space then they (armed drones)are slow, they are noisy, they are very easy to shoot right out of the sky. They are really not all that useful when it comes to states like Syria," said Audrey Kurth Cronin, a public policy professor at George Mason University.
The armed drones generally carry Hellfire missiles which have a 20-pound warhead, although some of the larger models can drop a 500-pound bomb.
By contrast, Tomahawk missiles carry a much bigger punch with a 1,000-pound warhead and fly just below the speed of sound, making them less vulnerable to air defenses.
The United States has four guided missile destroyers in the eastern Mediterranean Sea which can carry Tomahawk missiles, which have a range of about 1,000 miles.
If Syrian air defenses were hit in a U.S. strike, then drones could be used for surveillance and perhaps targeted strikes, although bringing them in at a later point could signal a more protracted engagement than Obama has suggested he is seeking in Syria, analysts said.
The U.S. Air Force has remotely piloted aircraft at the Incirlik Air Base in Turkey but U.S. officials would not discuss their mission.
They have been used in the past for surveillance along the Turkish-Syrian border, but it is not clear whether they are currently conducting surveillance inside Syria.
The United States also operates the RQ-170 Sentinel spy drone. The surveillance plane has a bat-wing, radar-evading shape, but its shortcomings were evident in 2011 when one crash-landed in Iran. Iran claimed to have taken control of the drone and forced it down, but U.S. officials denied that.
"We think of drones as a substitute for a stealth bomber or something - and they are not at all," said Daniel Byman, a Middle East security expert at the Brookings Institution.
While much has been made of Assad's network of Russian-made surface-to-air missiles, some experts say the effectiveness of the air defenses may have been overplayed.
"One of the problems that everybody has is that nobody really knows what the Syrian system can do," said Anthony Cordesman, a security analyst at the Center for Strategic and International Studies.
Editing by Alistair Bell and David Brunnstrom