The European Union condemned Wednesday the suspected use of chemical weapons by Syrian government forces as "totally unacceptable," demanding an immediate investigation.

EU foreign affairs head Catherine Ashton said charges by Syria's main opposition group that a chemical attack by the regime had left over 1,300 dead, "should be immediately and thoroughly investigated."

A UN mission in Syria to probe previous allegations of chemical weapons use "must be allowed full and unhindered access to all sites," Ashton said, according to a spokesperson.

"The EU reiterates that any use of chemical weapons, by any side in Syria, would be totally unacceptable," she said.

The authorities and all other parties in Syria "need to provide all necessary support to and cooperation with the mission's operations.

"We have also noted invitations by the opposition for the mission to visit rebel-held areas," Ashton added.