US Secretary of State Clinton held talks in Washington with EU top diplomat Catherine Ashton on May 17 and issued a renewed warning to Syria's leader.

"We will be taking additional steps in the days ahead," Mrs Hillary Clinton said after talks with.

The US and EU have already imposed sanctions on some members of Syrian President Bashar al-Assad's government.

Meanwhile, Damascus has denied reports that at least one mass grave has been found in the southern city of Deraa.

The reports were "completely untrue" and part of a "campaign of incitement", the Syrian interior ministry said.

Without being specific, Baroness Ashton said: "There will be a number of moves in the coming hours and days that you will see."

"If the [Syrian] government really does - as it keeps telling us it does - want to see some kind of change, it's got to be now," she added.

Mrs Clinton and Baroness Ashton declined to reveal what measures might be taken against Damascus.

Despite the sanctions on senior figures close to President Assad, both Washington and Brussels have so far stopped short of targeting him specifically.