BEIRUT, Lebanon - Any 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 United States said last week that intelligence indicated the Syrian regime had 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 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 was determined that the regime of President Bashar al-Assad had 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 that the administration was rethinking its opposition to arming the rebels, saying it was one of the options being considered.

In 2011, the United States and its NATO allies imposed a no-fly zone in Libya after Moammar Gadhafi's crackdown on an uprising there. 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 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 antiaircraft guns capable of engaging attacking aircraft at lower levels.

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.