Syrian National Coalition Secretary General Badr Jamous said that samples had been obtained from the area -- which he said had been besieged by government forces for months -- and were being sent outside Syria for analysis.
Jamous detailed the firing of rockets -- some with chemical warheads and others conventional weapons -- early in the morning into what he said was a heavily populated civilian area.
More than 1,300 were killed, most of them as a result of the use of chemical weapons against civilians, said Saleh -- all while the U.N. inspection team was only 10 kilometers (6 miles) away.
Jamous said the attack also injured more than 5,000 people.
The chemical component used has not yet been identified, Jamous said, but the number of casualties involved means the U.N. team should be moving in urgently to check the scene.
Graphic video footage showed rows of bodies without apparent injury, as well as people suffering convulsions or apparently struggling to breathe.
CNN could not immediately verify where or when the videos were recorded, and could not authenticate the number killed or injured.
Obama: 'Big event of grave concern'
In an exclusive interview with CNN's "New Day" anchor Chris Cuomo, U.S. President Barack Obama said preliminary signs point to a "big event of grave concern."
"It is very troublesome," he said. "That starts getting to some core national interests that the United States has, both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region."
U.S. military officials have updated options for a forceful intervention in Syria, a senior defense official tells CNN. The Joint Staff and U.S. Central Command last conducted a major update of options in April, in response to bipartisan pressure from congressional Republicans and Democrats.
Target lists for possible airstrikes were updated, including government buildings and military installations, but Syrian President Bashar al-Assad's forces and equipment "continue to move," and thus require flexibility in planning, the official said.
The planning, meant to give Obama "a current and comprehensive range of choices," also included updates on the potential use of cruise missiles, which would not require U.S. pilots to enter Syrian airspace, the official said.
No decision was made at a national security meeting Thursday at the White House, and no formal determination has been made on whether the Syrian government used chemical weapons, the official said.
On Friday afternoon, reporters asked White House deputy press secretary Josh Earnest about Syria.
"We've said the assistance we provide to the opposition is on an upwards trajectory, expanding in scope and in scale. We have long said that all options remain on the table when it comes to Syria.
"The president has indicated clearly that he does not foresee a situation in which American boots on the ground would be in the best interest of U.S. national security. But ultimately, that is the criteria he will use when he evaluates the proper course of action in this situation."
When reports of a possible chemical attack came in on Wednesday morning, U.S. Ambassador Samantha Power called for immediate consultations with the Security Council. The U.S. mission sent a joint letter to Secretary-General Ban, along with 36 other countries, that evening asking for an immediate investigation, a source with the mission told CNN.
Power was out of the office on personal travel at that time, but returned to her New York office on Friday, the source said.
The images of victims of the alleged attack, including many children, are "heartbreaking and sickening," Ban has said.