Washington - The White House disclosure that the Syrian government has twice used chemical weapons still leaves the Obama administration stuck with a limited choice of military options to help the rebels oust President Bashar Assad.
Arming the rebels runs smack into the reality that a military group fighting alongside them has pledged allegiance to al-Qaeda.
Establishing a no-fly zone poses a significant challenge as Syria possesses an air defence system far more robust than what the US and its allies overwhelmed in Libya two years ago.
President Barack Obama had declared that the Assad regime's use of chemical weapons in the two-year civil war would be "game changer" that would cross a "red line" for a major military response, but the White House made clear on Thursday that even a quick strike wasn't imminent.
Reflecting a strong degree of caution, the White House said the intelligence community assessed "with varying degrees of confidence" that the Syrian regime had used chemical weapons on a small scale.
The White House said in a letter to two senators that the "chain of custody" was unclear and that the determination was based on physiological samples.
The information had been known to the administration and some members of Congress for weeks despite public pronouncements from the White House.
‘Up military pressure on Assad’
The revelation on Thursday strengthened proponents of aggressive military action, who challenged the administration to act and warned that going wobbly would embolden Assad.
Yet it also underscored the difficulties of any step for war-weary lawmakers horrified by a conflict that has killed an estimated 70 000 but guarded about US involvement in a Mideast war.
"There's no easy choice here," said Democratic Sen. Claire McCaskill, a member of the Armed Services Committee. "All the alternatives are flawed. It's just finding the least flawed among them that will get Assad out."
The next move on Syria was high on the agenda for Obama's meeting on Friday with King Abdullah II of Jordan, as the US ally has struggled with the influx of hundreds of thousands of refugees escaping the Syrian violence.
Vice President Joe Biden and Abdullah discussed the best path to "a peaceful, democratic post-Assad Syria where moderates are empowered" on Thursday.
"I think it's important for the administration to look for ways to up the military pressure on Assad," said Democratic Sen. Carl Levin, chairperson of the Armed Services Committee.
‘Right weapons in right hands’
One of the most powerful of the rebel groups in Syria is Jabhat al-Nusra, which recently declared its affiliation with al-Qaeda.
Last December, the State Department designated the group a terrorist organisation, and the administration's opposition to directly arming the Syrian opposition stems from concerns about the weapons ending up in the hands of Islamic extremists.
Arming the rebels, said Republican Sen. Lindsey Graham, is a "lot harder than it was before."
"We've gotten to the point now where the opposition has been affected by the radicals," Graham said in an interview.
"Right weapons in right hands is the goal. The second war is coming. I think we can arm the right people with the right weapons. There's a risk there, but the risk of letting this go and chemical weapons falling into radical Islamists' hands is the greatest risk."
Several lawmakers, including Republican Sen. John McCain, have called for the US to create a narrow, safe zone inside Syria, along its border with Turkey.
200 US troops deployed
Either a safe zone or a no-fly zone would require neutralising Syria's air defences. According to a report by the Institute for the Study of War, Syria's largely Soviet-era air defence system includes as many as 300 mobile surface-to-air missile systems and defence systems, and more than 600 static missile launchers and sites.
"You can establish it [safe zone] by taking out their aircraft on the ground with cruise missiles and using the Patriot [missile] also. No American manned aircraft in danger," McCain said.
The US has taken only minimal military steps so far, limiting US assistance to nonlethal aid, including military-style equipment such as body armour and night vision goggles.
The US has deployed about 200 troops to Jordan to assist that country's military, and participated in Nato's placement of Patriot missile batteries in Turkey near the border to protect against an attack from Syria.
It's unclear, however, what arming the rebels or patrolling a no-fly zone over Syria would accomplish.
"The options are all bad," says Aram Nerguizian, senior fellow at the Center for Strategic & International Studies. "Arming the opposition doesn't do anything regarding chemical weapons or solving proliferation concerns in Syria."
Targeting a facility, he added, might send a message to the Assad regime. But it does little to address the larger direction of the civil war, which is tilting back toward government forces again after a counteroffensive.
"Here's one thing you can do," argues Andrew Tabler at the Washington Institute for Near East Policy in a similar vein. "If they load this stuff into bombs or mix the stuff, we can hit it," he said, but agreed that wouldn't eliminate the larger stockpiles or address the larger context of a conflict that is destroying Syria.
In testimony to Congress last week, Gen. Martin Dempsey, the chairperson of the Joint Chiefs of Staff, was asked whether he was confident that US forces could secure the chemical weapons caches within Syria.
"Not as I sit here today, simply because they've been moving it and the number of sites is quite numerous," Dempsey said.
Tabler pointed to the Israeli attack earlier this year on a Syrian weapons convoy going to Hezbollah as an example of a possibly targeted US intervention.
He said the question of arming the rebels should be looked at beyond chemical weapons use, considering the 200 Scud missiles that have been launched by Assad's regime in the last five months and the government's ongoing escalation "all over the place."
Even if US interests aren't immediately affected, they could be over time.
"Syria isn't Vegas," Tabler said. "What happens in Syria doesn't stay in Syria. Where do these chemical weapons all go?"