BEIRUT (AP) -- International military action against Syria's government over its alleged use of chemical weapons would run up against one of the Middle East's most formidable air defenses, a system bolstered in recent years by top-of-the-line Russian hardware.
The U.S. said last week that intelligence indicates the Syrian regime has likely used the deadly nerve agent sarin on at least two occasions in the civil war. That assessment has increased pressure for a forceful response from President Barack Obama, who has said the use of chemical weapons would cross a "red line" and carry "enormous consequences."
Obama has tried to temper expectations of quick action against Syria, saying he needs "hard, effective evidence" before making a decision. But he has also said that if it is determined that the regime of President Bashar Assad has used such weapons, then "we would have to rethink the range of options that are available to us."
Defense Secretary Chuck Hagel told a news conference Thursday the administration is rethinking its opposition to arming the rebels, saying it is one of the options being considered along with its allies in the more than 2-year-old conflict.
In 2011, the U.S. and its NATO allies imposed a no-fly zone in Libya after Moammar Gadhafi's brutal crackdown on its uprising. The allied air campaign, which received U.N. backing, played a major role in the rebels' victory in Libya's eight-month civil war.
While NATO quickly knocked out Libya's air defenses, experts warn that Syria's capabilities are far more sophisticated and its system is far more extensive than Gadhafi's was.
"In the case of Libya, the system had deteriorated completely already before the outbreak of the conflict due to the fact that Gadhafi had not invested so much in his air defense," said Pieter Wezeman, a senior researcher at the Stockholm International Peace Research Institute. "In the case of Syria, it's quite different."
Syria, experts say, possesses one of the most robust air defense networks in the region, with multiple surface-to-air missiles providing overlapping coverage of key areas in combination with thousands of anti-aircraft guns capable of engaging attacking aircraft at lower levels.
Six years ago, the system was showing signs of neglect.
In 2007, Syria's aging Soviet-supplied air defense system received a shock when Israeli jets bombed a suspected nuclear reactor site along the Euphrates River in northeastern Syria. The attack proved deeply embarrassing and provided a jolt to the Assad regime, which responded by making a concerted push to upgrade its air defenses, and turned to the country's traditional arms provider, Russia, for help.
Moscow, which has been the source of most of Syria's military hardware since Assad's father and predecessor, Hafez, courted the Kremlin decades ago, was more than happy to oblige.
It provided Syria with new systems, such as 36 Pantsyr mobile surface-to-air missile systems and at least eight Buk-M2E mobile SAMs. The Pantsyrs, considered particularly effective against attacking aircraft, feature a combination of 30mm cannons paired with a radar and anti-aircraft missiles all on the same vehicle.
At the same time, old SA-3s were upgraded to Pechora-2Ms -- essentially a new and much more capable system.
There have also been persistent rumors that Syria acquired the advanced, Russian-built S-300 anti-aircraft missiles, considered to be the cutting edge in aircraft interception technology, although there are doubts about whether Damascus actually has them.
Moscow had refused to deliver the systems, but there have been unconfirmed reports that other nations may have sent Syria the missiles, which could make any aerial intervention very costly for the attackers.
"It's certainly the kind of system that if delivered would increase the risks for Israel, the USA or anyone else would they want to intervene militarily," Wezeman said.
Separately, Syria also has obtained from Russia the mobile Bastion-P land-based coastal defense systems, including Yakhont anti-ship missiles capable of sinking large warships, including aircraft carriers.
Russia has stood by Damascus since the uprising against Assad began in March 2011, shielding it from U.N. sanctions and continuing to supply the Syrian military with air defense components. Its support has not faded, even as the death toll in the conflict passed 70,000.
In February, Anatoly Isaikin, the head of the state arms trading agency Rosoboronexport, said that since there are no sanctions against shipping weapons to Syria, Russia was still fulfilling its contract obligations.
"These aren't offensive weapons," he said. "We are mostly shipping air defense systems and repair equipment for various branches of the military."
Like all of the Syrian military, the country's air defense system undoubtedly has suffered damage since the conflict became a civil war. Rebels have captured large swaths of northern Syria and have made a bridgehead in the south along the border with Jordan. Hydroelectric dams, cities and military bases have fallen into rebel hands.