United Nations human rights investigators said Sunday they have gathered testimony from outside Syria suggesting rebels, not Bashar Assad's regime, may have used chemical weapons.

“Our investigators have been in neighboring countries interviewing victims, doctors and field hospitals and, according to their report of last week which I have seen, there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas, from the way the victims were treated,” Carla Del Ponte, a member of the independent commission of inquiry on Syria, told Swiss-Italian television. “This was use on the part of the opposition, the rebels, not by the government authorities.”

The allegations will likely make it harder for the Obama administration to justify taking a more active role in the two-year-old civil war on the side of the opposition. The administration has said in recent days that chemical weapons appeared to have been used in Syria, which would violate the “red line” Obama set for Assad's forces.

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The Syrian government has denied using chemical weapons and initially invited UN inspectors to probe a March attack in the village of Khan al-Asal near Aleppo, saying rebels used chemical weapons in that incident. The regime has refused to allow the inspectors to enter the country, however, after France and Britain demanded that they be allowed to investigate other reported sites of chemical weapons use, notably in the village of Ataybah near Damascus on March 19 and in Homs last December.

Calls for a greater U.S. role – such as arming vetted rebels and operating a no-fly zone – grew over the weekend after airstrikes, apparently by Israeli warplanes, revealed weaknesses in Syria's vaunted air-defense system. Some lawmakers, however, have long cautioned that the opposition is heavily influenced by Islamists and have cautioned that any government that replaces Assad may be antipathetic to both Israel and the United States.