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.
"Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law. Such a crime against humanity should result in serious consequences for the perpetrator."
Ban said the situation in Syria, where rebels have been fighting al-Assad's forces for more than two years, continues to worsen. The death toll has surged past 100,000, he said.
In a statement Friday, European Union foreign policy chief Catherine Ashton backed the United Nations' request for "a thorough, impartial and prompt investigation into these alleged chemical attacks."
"The international community must now urgently show a united face and ensure that a credible and thorough investigation can be carried out," she said.
Iranian President Hassan Rouhani condemned the use of chemical weapons, particularly in Syria, Iran's state-run news agency, IRNA, said Saturday.
"The conditions ruling Syria, in which a large number of innocent people are wounded or killed by the chemical elements, are regrettable," he said, according to semiofficial news agency, Fars.
A million child refugees
Two U.N. agencies announced Friday that the number of child refugees from Syria has passed a landmark threshold, with 1 million forced to flee during the conflict. They make up half of all refugees from the country.
About 740,000 of the children registered are younger than 11, U.N. children's agency UNICEF said. Most have arrived in Lebanon, Jordan, Turkey, Iraq and Egypt, with some families also heading to North Africa and Europe.
"This one millionth child refugee is not just another number," said UNICEF Executive Director Anthony Lake. "This is a real child ripped from home, maybe even from a family, facing horrors we can only begin to comprehend."