"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."
Inside Syria, about 7,000 children have been killed during the conflict, according to the Office of the High Commissioner for Human Rights, while another 2 million children have been internally displaced.
U.N. High Commissioner for Human Rights Antonio Guterres told CNN there was the risk of a lost generation in Syria and many of the children caught up in the conflict are showing a high level of trauma.
"I've seen many that do not speak any more, I've seen some with broken sleeping, that have enormous difficulties, some with behaviors that are very challenging and very strange," he said.
The apparent presence of many small children among the victims of Wednesday's alleged attack will add to concerns about the safety of Syria's most vulnerable citizens.
An al-Assad government spokesman said any claims it used chemical weapons are "illogical and fabricated."
Syrian Information Minister Omran Zoabi told state TV the claim was timed by the opposition to coincide with the U.N team's visit and came as government forces were making gains on all sides against the rebels.
In the streets of government-controlled Damascus, many people said they do not believe the government resorted to the use of nerve agents. The CNN team is in Syria is on an officially approved trip.
"The government would never use chemical weapons because Bashar al-Assad is part of the country, he grew up here, they are Syrians," one man told CNN.
Another said he believed that if anyone was hit, it was members of the rebel Free Syrian Army.