Updated 2:56 p.m.

Opposition activists in Syria are claiming that the embattled regime of Bashar Assad gassed rebel forces in the battleground city of Homs on Sunday. U.S. officials tell Danger Room that they are skeptical about the rebels' chemical weapon claims, however.

Al Jazeera reported that seven people died after inhaling a gas sprayed by government forces in a part of Homs held by the rebel Free Syrian Army. "We don't know what this gas is but medics are saying it's something similar to sarin," rebel Raji Rahmet Rabbou told the Qatar-based news organization.

The "poisonous material" was deployed by government warplanes, Haaretz reported, citing a rebel statement. The Assad regime, meanwhile, is blaming the rebels for the attack.

Al Jazeera posted two videos it said were obtained from "a field clinic in the city." The graphic videos appear to depict gasping victims of what could be construed as a nerve agent attack. However, the origins and contents of these videos have yet to be verified by other sources.

U.S. officials note that several things about the video are inconsistent with a sarin strike. There are complaints of strong smells; sarin is odorless. There are reports that the victims inhaled large amounts of the chemical; a minuscule of amount of inhaled sarin can be fatal.

"It just doesn't jibe with chemical weapons," one U.S. official tells Danger Room.

In fact, the symptoms shown in these videos might have been caused by other chemicals – possibly chlorine, phosgene, or cyanogen chloride, according to one independent review of the clips (.pdf). Or we might simply be seeing a severe asthma attack.

The specter of chemical warfare has long loomed over the brutal Syrian conflict, which rebels claimed has killed no fewer than 37,000 people. As early as this summer Assad's regime warned it might deploy its 500-ton chemical stockpile. "There was a moment we thought they were going to use it — especially back in July,” a U.S. official told Danger Room. "But we took a second look at the intelligence, and it was less urgent than we thought."

The relief was short-lived. Assad began trying to expand his arsenal with fresh sarin precursor materials – specifically, isopropanol and methylphosphonyl difluoride. U.S. and allied agents blocked at least some of the new acquisitions, but there was little they could do about the existing stockpile.

Three weeks ago U.S. surveillance spotted special Syrian military units mixing the precursors and prepping sarin warheads for possible use. "Physically, they’ve gotten to the point where the can load it up on a plane and drop it," an official told Danger Room at the time.

Washington and its allies have repeatedly said they would not tolerate a chemical attack. "This would cross a red line and those responsible would be held to account," Secretary of State Hillary Clinton warned previously.

Even Sergei Lavrov – foreign minister of Assad's ally, Russia – called the use of chemical weapons "political suicide."

Now the question is: Has the regime actually taken that step? Are the rebels confused? Or is this an opposition effort to further discredit the regime?