Added Ahmed Moaz al-Khatib, head of the Syrian National Coalition: "First, the Syrian regime lies most of the time.... We are against any use of any chemical weapons from any side."
Syrian rebels accused the government of firing a rocket at a police school west of Aleppo, but the rocket landed in the wrong area, striking an area under control by government forces.
The state-run Syrian Arab News Agency, which reported that most of those killed were civilians, showed photos of people being treated in hospitals on its website.
But Louay Almokdad, political and media coordinator for the Free Syrian Army, told CNN that the rebels lack access to chemical weapons and surface-to-surface missiles. He confirmed injuries in an attack in the town, but said it was carried out with a missile possessed only by the government.
"The area that was targeted is under rebels' control, so it is quite absurd that the regime would accuse us of attacking our own people," he said.
"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.