The US still believes that Syrian President Bashar al-Assad would not be part of a transitional government in the country, US Secretary of State John Kerry has said.

Mr Kerry made his comments to reporters on the sidelines of a meeting with Jordanian Foreign Minister Nasser Judeh in Rome.

"The foreign minister will work with us, as they have, to try to bring all the parties to the table so that we can effect a transition government by mutual consent of both sides, which clearly means that in our judgment President Assad will not be a component of that transitional government," Mr Kerry said.

Meanwhile, Britain said it believed it was "very likely" that the Syrian government had used chemical weapons, but said it had "no evidence to date" that rebels fighting to topple Syrian President Assad had used such arms.

When asked about reports that rebel forces had used the banned nerve agent sarin, a spokesman for Prime Minister David Cameron said: "Our assessment is that chemical weapons use in Syria is very likely to have been initiated by the regime. We have no evidence to date of opposition use."