The European Union on Friday reiterated a request to Damascus to enable a UN chemical weapons probe in Syria after the United States said for the first time that the regime has probably used such weapons.
"We hope there will be a United Nations investigation inside Syria to hopefully shed some light on what has really happened," a spokesman for the EU's top diplomat Catherine Ashton said after being queried over the EU stand.
"The bottom line is this would of course be clearly unacceptable" if proven, said spokesman Michael Mann.
Asked if Brussels remained unconvinced over the allegations of chemical weapons use, he added: "We are still wanting more monitoring to be done ... We don't want to be definitive on this until we have some definitive evidence."
"The most important thing is for the United Nations to carry out an investigation inside the country."
On Thursday, the United States said for the first time that Syria had likely used chemical weapons against rebel forces, but emphasised that spy agencies were still not 100 percent sure of the assessment.
US intelligence services had been investigating reports that Assad's forces had used chemical arms -- a move President Barack Obama has warned would cross a "red line".
UN leader Ban Ki-moon on Thursday renewed an "urgent call" for Syria to let inspectors into the country.
Syria had asked for a UN investigation into its claims that opposition rebels had used chemical weapons but has since refused to let a UN team waiting in the region into the country.