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 people 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.
British Foreign Secretary William Hague expressed concern at the reports, saying it "is a very important matter to pursue (and)... We are pursuing this with urgency."
His German counterpart Guido Westerwelle demanded that the UN chemical weapons team in Syria should "immediately have the opportunity to verify the allegations comprehensively."
The charges were "very serious and alarming," Westerwelle said.
French Foreign Minister Laurent Fabius, speaking in Brussels, said if proven the use of chemical weapons would "not only be a massacre, but also an unprecedented atrocity".
Swedish Foreign Minister Carl Bildt called on Syria to grant "urgent" access to UN investigators to determine whether the regime has used the weapons.
The main Syrian opposition group said President Bashar al-Assad had attacked rebel areas near Damascus on today using chemical weapons.
The charges could not immediately be verified and were vehemently denied by the authorities.
There have been frequent claims by the rebels of the use of chemical weapons by the army, particularly in Damascus province and Homs in central Syria.