A Damascus opposition monitoring group said 494 people have been killed in the attack, citing data from medical centers in the Syrian capital.
If confirmed, it would be by far the worst reported use of chemical weapons in the two-year-old Syrian conflict.
“I have seen with grave concern the reports of the possible use of chemical weapons by the Syrian regime,” Ashton said in a statement, adding that “such accusations should be immediately and thoroughly investigated by the United Nations expert mission which arrived recently in Syria”.
"I reiterate that any use of chemical weapons, by any side in Syria, would be totally unacceptable,” she stressed.
Ashton urged full cooperation by Syrian authorities in the investigation, saying the UN mission “must be allowed full and unhindered access to all sites on the Syrian territory”.
A team of UN chemical weapons experts arrived in Syria three days ago.
“The Syrian authorities as well as all other parties in Syria need to provide all necessary support to and cooperation with the mission's operations,” Ashton said on Wednesday as EU foreign ministers were gathering in Brussels for a meeting dedicated to the crisis in Egypt.
Speaking on the sidelines of the EU meeting in Brussels, French Foreign Minister Laurent Fabius said the UN mission in Syria to must immediately investigate the alleged chemical attack.
"What France is asking is that this mission which is in place must investigate immediately," he told reporters on Wednesday. If confirmed, Fabius said the attack would "not only be a massacre, but also an unprecedented atrocity".
He added however that the allegations by the Syrian opposition were “not yet verified” and that evidence needed to be firmed up before drawing further conclusions.
“We know that the regime has chemical weapons, and so first, their use must be established,” Fabius said.
Britain said it would raise the issue at the United Nations Security Council and called on Damascus to give UN inspectors access to the site.
"I am deeply concerned by reports that hundreds of people, including children, have been killed in airstrikes and a chemical weapons attack on rebel-held areas near Damascus," British Foreign Secretary William Hague said in a statement.
Hague said reports of the attack, which has been denied outright by the Syrian government, remained uncorroborated and that Britain was urgently seeking more information.
"But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria," he said, adding that Britain would try to hold to account anyone who used chemical weapons or ordered their use.