"The Assad regime possesses chemical agents and they already used weapons of mass destruction against its own people, so we do expect the worse from this brutal psychopathic regime," he said.

An activist Facebook page said the location was between rebel-held and regime-held territory, and it appeared that the blast hit mostly Syrian soldiers and some civilians in a regime-held area.

As for Ateibeh, the shelling caused deaths and many injuries, "including suffocating and nausea cases and headache, vomiting and hysteria cases," the opposition Local Coordination Committees of Syria said.

There was no immediate government comment about Ateibeh.

International reaction: Shock, concern, skepticism

The international community is looking into the reports. The Russian Foreign Ministry, citing information from Damascus, said chemical weapons were used by the armed opposition, causing deaths and injuries.

"We believe the new incident is an extremely alarming and dangerous development in the Syrian crisis," the Russian ministry said. "Russia is seriously concerned about the fact of (weapons of mass destruction) coming into the hands of militants, which makes the situation in Syria even worse and brings the confrontation in the country to a new level."

The Obama administration is carefully investigating the reports, White House spokesman Jay Carney said.

Carney said determining what happened is a top priority.

"There will be consequences, and they will be held accountable," Carney said, passing along the president's comment.

"We also consider a red line the proliferation of chemical weapons to other actors by the regime," he added.

Obama will be discussing the Syrian crisis during his visit this week to the Middle East, where it will be a topic of conservation with Israeli, Jordanian and Palestinian leaders.

The British Foreign Office is also checking on the reports.

"The use of chemical weapons would be abhorrent and universally condemned. The UK is clear that the use or proliferation of chemical weapons would demand a serious response from the international community and force us to revisit our approach so far," a spokesman said.

Two senior U.S. officials said they don't believe the rebels used chemical weapons and suggested the government itself may have manufactured the incident to preserve the ability to use them in the future.

"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.