With technical support from Russia, Syria has bolstered its air defenses, posing a threat to US aircraft if America decides to intervene in the war, a US official said Monday.

The official confirmed a report that first appeared in the Wall Street Journal.

Word of the upgraded defenses takes on new urgency given US assertions that Syria may have used chemical weapons against rebel forces -- an assessment that will test President Barack Obama's repeated statement that such a move would be a "game changer" for Washington.

"The Syrians have stepped up their efforts in recent years to bolster their air defenses, particularly after the covert nuclear facility they were building was destroyed," said the official, speaking to AFP on condition of anonymity.

The official was alluding to a nuclear reactor destroyed in an Israeli air raid on September 6, 2007.

The regime of President Bashar al-Assad relied on technical support from Russia to upgrade its air defense system, which dates back to the Soviet era, this official said.

But the United States rarely interfered because it viewed Iran as the region's larger threat, the Journal said. And in the early part of the Obama administration, the United States sought to improve ties with both Russia and Syria, it added.

Russian technicians are on hand with many of the anti-aircraft defense units to provide assistance and repair broken equipment with parts imported from Russia, the Journal said.

Quoting a US intelligence assessment, the Journal said that in August 2008 Russia began shipping 36 SA-22 Pantsir S1 units to Syria. They combine surface-to-air missiles and an anti-aircraft gun, and are mounted on combat vehicles and thus mobile.

In 2009, Moscow started upgrading Syria's outdated analogue SA-3 surface-to-air missile system, turning them into a system which is mobile and digital and capable of taking out cruise missiles.

Russia, one of the few countries that still support Assad, also helped Syria modernize its SA-5 system. This one fires missiles with a range of 175 miles and could hit US planes taking off from Cyprus, a key NATO base, the paper said.

Republicans are calling for US action of some kind against Syria in light of the new reports that Damascus may have used chemical weapons against rebel forces.

One military option -- not under consideration at this point -- would be to establish a no-fly zone, which would involve taking out Syria air defenses.