(Repeats for wider distribution)
By Matt Spetalnick and Phil Stewart
WASHINGTON, Aug 28 (Reuters) - American forces face formidable challenges as President Barack Obama considers an air assault on Islamist fighters in Syria, including intelligence gaps on potential targets, concerns about Syria's air defenses and fears that the militants may have anti-aircraft weapons, current and former U.S. officials say.
The Pentagon began preparing options for an assault on Islamic State fighters after the militants last week posted a gruesome video showing the beheading of American photojournalist James Foley. Deliberations by Obama's national security team on expanding the campaign against Islamic State from Iraq into neighboring Syria gathered pace in recent days, officials say.
While it is unclear how soon strikes might be launched, Obama's go-ahead for aerial reconnaissance over Syria has raised expectations he will approve the attacks rather than back off as he did last year after threatening to strike Syrian President Bashar al-Assad's forces.
Any air offensive would likely focus on Islamic State's leadership and positions around the city of Raqqa in their stronghold of eastern Syria, and border areas that have served as staging grounds for Islamist forces that have swept into Iraq and taken over a third of the country.
But every option carries significant risk.
"There are all kinds of downsides and risks that suggest air strikes in Syria are probably not a great idea," said Aaron David Miller, a former Middle East adviser under both Republican and Democratic administrations. "But that doesn't mean they won't happen anyway."
Efforts to hit the right targets in Syria will be more difficult than in Iraq, hindered by a shortage of reliable on-the-ground intelligence, in contrast to northern Iraq where Iraqi and Kurdish forces provided intelligence.
U.S.-backed moderate rebels who could provide intelligence in Syria have yet to coalesce into a potent fighting force. It is unclear, for instance, if they can provide forward spotters needed to help guide any air strikes in territory held by Islamic State.
RUSSIAN-BUILT AIR DEFENSES
Syria's Russian-built air defense system is another concern. It remains largely intact more than three years into the country's civil war.
Assad may opt not to use it, mindful that he could benefit from a U.S. assault on Islamic State. He has struggled to fend off advances by the radical offshoot of al Qaeda, which has taken three Syrian military bases in northeast Syria in recent weeks, boosted by arms seized in Iraq.
He could also face U.S. retaliation for any Syrian government interference in a U.S. air campaign.
Of greater concern to Western military planners is anti-aircraft weaponry Islamic State fighters might have acquired.
"Flying aircraft over Syria is very different than in Iraq," said Eric Thompson, senior strategic studies analyst at the Center for Naval Analyses, which advises the U.S. military as part of the CNA Corp think tank in Virginia. "There are more sophisticated air defenses, some in the hands of ISIS," he added, using an alternative name for Islamic State.
In a recent report, Small Arms Survey, an independent research group based in Geneva, detailed a range of shoulder-launched missile systems in the hands of the militants. Known as MANPAD, or man-portable air defense systems, some were apparently stolen from government stockpiles while others were supplied from outside sources in other countries.
The Pentagon has publicly conceded it has less-than-perfect information about the movements and capabilities of Islamic State fighters, a limitation reflected in a failed attempt by U.S. special forces to rescue Foley in July.
Intelligence gaps raise the risk of civilian casualties from any U.S. air strikes in Syria, especially given that the militants are highly mobile and intermingle with the civilian population in urban areas like Raqqa.
From unmanned armed drones to powerful Stealth bombers, a wide range of U.S. airpower is at Obama's disposal, including possible missiles fired from warships at sea or from aircraft flying outside Syria's borders.
Drones, Obama's weapon of choice in the fight against al Qaeda in Pakistan and Yemen, could also be used, but possibly more for surveillance than missile strikes. Given the risk of missed targets and civilian casualties, U.S. forces typically prefer to operate drones in tandem with intelligence operatives on the ground.
Islamic State leaders' use of encryption in communications is highly sophisticated and hinders efforts to track them, according to U.S. officials familiar with the group's tactics. As a result, Islamic State leaders such as Abu Bakr al-Baghdadi are expected to be hard to find. (Additonal reporting by Mark Hosenball and Peter Apps. Editing by Jason Szep and Peter Henderson)