By Phil Stewart and Peter Apps
WASHINGTON (Reuters) - Israeli missiles breached Syria's vaunted air defense system over the weekend, but that offered little comfort to U.S. military planners weighing the risks of any intervention against President Bashar al-Assad's forces.
With some of the possible U.S. military options in Syria involving a need for air power, the Pentagon remains concerned about Assad's ability to shoot down enemy aircraft with surface-to-air missiles, particularly in a sustained campaign.
President Barack Obama has resisted pressure to deepen involvement in Syria's civil war and has stopped short of even limited steps like arming the anti-government rebels.
If the United States did become more embroiled in Syria - perhaps in reaction to Damascus using chemical weapons - and wanted to wage a large air campaign there it would likely first need to take out Syria's Russian-made air shield.
While the effectiveness of Syria's aging air force is unclear, most experts believe its air-defense missile system, considerably upgraded after a 2007 Israeli strike on a suspected nuclear site, remains more potent than any the United States has faced since it bombed Serbian forces in 1999.
"These recent events have not changed our assessment of the sophistication of the Syrian air defense system," said a senior U.S. official.
That said, the United States does indeed have the power to wipe out Syria's air defenses.
Syria has little or no protection against hard-to-stop weapons in the U.S. arsenal like B-2 stealth bombers or ship- and submarine-launched cruise missiles. Still, it would require a huge assault involving cruise missiles, and jets possibly flying either from aircraft carriers or bases in neighboring countries.
Israeli jets managed to avoid Syrian defenses twice again in recent days, but the raids were surprise strikes that experts said would have been difficult to defend against. U.S. officials said last week the Israelis did not even enter Syrian airspace in Friday's bombing, firing missiles instead from the skies over neighboring Lebanon.
U.S. jets would be far more at risk if they tried to impose a no-fly zone over Syria or to protect "safe zones" on the ground, which would almost certainly require operations over the country for long periods of time.
"There is a huge difference between conducting a strike and implementing a no-fly zone," a second U.S. official said.
The Pentagon estimates than Syria has five times more air defenses than those that existed in Libya, where the United States helped establish a no-fly zone in 2011. They are also far more densely packed and sophisticated.
In Libya, there were no Western casualties. But the risks are higher in Syria and it's unclear whether the war-weary American public - exhausted by the conflicts in Iraq and Afghanistan - would tolerate U.S. casualties.
Before the Syrian civil war began, Western analysts estimated that Assad had around 25 air-defense brigades with some 150 surface-to-air missile launchers.
The first U.S. official noted that Syrian air defenses have been strengthened in recent years. Still, the extent to which its civil war may have degraded those defenses is unclear.
Many of Syria's anti-aircraft missiles are mobile, which means Assad's forces could choose to locate them near schools or apartment buildings, hoping U.S. forces might avoid targeting them for fear of causing civilian casualties. The density of the defenses raises the risk of civilian deaths.
The shooting down of a Turkish F4 Phantom reconnaissance jet as it neared the Syrian coast last year demonstrated Syria's quick-reaction air defenses. But the United States and its allies have options that could make such action safer.
According to one Western defense planner, taking down the entire Syrian system would involve a heavy opening salvo of cruise missiles and then air strikes. The Center for Strategic and International Studies think tank estimates that such an assault would take the combined combat power of at least two U.S. aircraft carriers, although U.S. and allied aircraft could also be based at nearby land bases in Turkey, Jordan and Cyprus.
Obama would be unlikely, however, to go forward with any operation alone, looking to allies like Britain and France to also contribute in the event the United States - despite its extreme reluctance to get involved militarily - found itself forced to take direct action in the Syrian conflict.
Syria is believed to lack any significant defenses against U.S. cruise missiles from reaching their targets or detect and intercept B-2 stealth bombers, which could be used to damage Syrian airfields.
"If you were to fly a B-2 over one of their airfields, you could probably make it so that it would be out of service for awhile," says Air Vice Marshal Michael Harwood, a retired Royal Air Force officer who was British defense attache to Washington until 2012.
Given the large number of casualties in Syria that are caused by artillery, U.S. forces might need also to strike government artillery if they wanted genuinely to protect any rebel-held "safe zones" on the ground.
General Martin Dempsey, the top U.S. military officer, told the Senate last month: "The safe zone is only safe if you ensure its safety."
"You have to control the terrain at some distance beyond it in order to do that."
The Predator, Reaper and Global Hawk-type drones on which Washington has come to rely in Afghanistan - where U.S. air power is virtually unchallenged - would be of little use until Syrian air defenses were neutralized. Those aircraft were not designed to defend themselves from attack.
(Editing by Alistair Bell and Jackie Frank)