"The regime is using (the claims) as a pretext for their own possible use," one of the officials said. "The opposition has no such weapons."

The officials said they could not confirm a rebel claim that the regime used some type of agent on its own people in order to blame the rebels but could not rule it out. Officials pointed to previous claims that chemical weapons were used, which, after extensive investigation, were unsubstantiated.

The Syrian Foreign and Expatriates Ministry sent two letters Tuesday to the United Nations warning of the dangers of chemical weapons getting into the hands of al Qaeda-linked opposition groups.

The Syrian government did not use chemical weapons against residents of Homs in a December attack, a U.S. State Department investigation showed but did apparently misuse a riot-control gas in the incident, according to senior U.S. officials.

The investigation stemmed from allegations inside Syria about the use of chemical weapons during the attack on the city of Homs on December 23. The officials said the State Department launched a probe from its consulate in Istanbul after doctors and activists reported dozens of victims suffering from nervous system, respiratory and gastrointestinal ailments after inhaling the gas.

Military analysts believe the Syrian government may have one of the largest stockpiles of chemical weapons in the world. Specifically, the supply could include sarin, mustard and VX gases.

Arming the rebels

Dissidents inside and outside Syria have called for the United States to take a greater role in helping Syrian rebels, including supplying arms.

So far, the Obama administration has donated nonlethal and humanitarian aid.

But Secretary of State John Kerry said the United States would not stand in the way of its allies' arming Syrian rebels.

Kerry acknowledged the need to change the military "imbalance" on the ground in order to change al-Assad's "calculus."

"Right now, President Assad is receiving help from the Iranians, he's receiving help from al Qaeda-related, some elements, he's receiving help from Hezbollah, and obviously some help is coming in through the Russians," Kerry said. "If he believes he can shoot it out, Syrians and the region have a problem and the world has a problem."

Members of the rebel Free Syrian Army have said they've received shipments from some countries and seized and purchased weapons from government troops. But al-Assad's forces have heavy weaponry and warplanes.

Last week, the French foreign minister said he wanted to lift a European Union arms embargo and start arming rebels.

"We must go ahead and allow the Syrian people to defend themselves against this bloodthirsty regime. It's our responsibility to help the (opposition) Syrian National Coalition, its leaders and the Free Syrian Army by all the possible means," Laurent Fabius wrote in an op-ed for the French newspaper Liberation.

"If not, the slaughter will continue, and there will not be any other possible outcome but to strengthen the most extreme groups and the collapse of Syria with devastating consequences for the country itself and the region."

In February, the European Union renewed its arms embargo on Syria for three months -- but amended it to allow greater nonlethal support and technical assistance to help protect civilians.

The latest EU arms embargo is set to expire in May. Member countries could renew it, add amendments or veto it.

A new opposition leader

A Syrian opposition alliance elected Ghassan Hitto, an information technology executive and U.S.-educated Kurdish businessman, to lead its provisional government.

The National Coalition for Syrian Revolutionary and Opposition Forces made the announcement Tuesday.

The contingent government's formation should assuage concerns from the West -- particularly the United States -- about who would lead Syria should al-Assad be deposed, the Syrian American Council said.

"This question has now been answered," the group said.

For two years, the lack of a clear alternative to al-Assad's government has hampered the opposition's efforts in gaining more international support. Some say the absence of an alternative leader has helped prolong the bloodshed.

It didn't take long for Hitto to declare what many in the opposition have said: "There will be no dialogue with the Assad regime."