US and EU seek to pressure Syria

"We will be taking additional steps in the days ahead," US Secretary of State Hillary Clinton said after talks with EU top diplomat Catherine Ashton.

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.

In a separate development, online activists seeking reform in Syria have called for a nationwide strike on Wednesday, although it is not clear how much support any protest would command.

Clinton held talks in Washington with Baroness Ashton on Tuesday and issued a renewed warning to Syria's leader.

Syrian security forces launched a crackdown in Deraa in April

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.

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.

Syria's unrest has lasted some two months now, beginning in mid-March in the city of Deraa.

The city became an epicentre of the unrest after security forces launched a major operation to crush any dissent there. Dozens of people are believed to have been killed and hundreds arrested.

On Tuesday, the Syrian interior ministry admitted that five bodies had been found on Sunday in the Bahar area of Deraa, and that the deaths were being investigated.

But it dismissed allegations of a mass grave as part of a campaign of incitement and fabrication by the international media.

Syrian human rights organisations, however, brushed aside the denials and insisted that 24 bodies were dug out of a shallow grave. The victims included a 62-year-old man, four of his children, a woman and another child, they said.

They also said a total of 41 other corpses were found strewn around wheat fields near the towns of Jassem and Inkhil, to the north of Deraa, both of which were attacked by security forces recently.

Amnesty International called on the Syrian authorities to launch "a prompt, impartial investigation into reports that a number of bodies were unearthed" and "into how those deaths occurred".

President Assad is reported to have personally assured the UN Secretary General, Ban Ki-moon, that a humanitarian mission could go to Deraa to assess the situation. It has not been able to despite repeated requests.

The BBC's Jim Muir in Beirut says amateur video shows Syrian army tanks still inside the city, despite several announcements that they have been withdrawn.

The army is also still in action at the latest hotspot - the town of Tal Kalakh, north-west of Damascus, where a crackdown has sent hundreds of civilians fleeing across the nearby border into Lebanon, he adds.

The Syrian military announced that eight soldiers or police were killed in the latest incidents, which it blamed on "armed criminal gangs". The government says more than 120 soldiers and police have died.

Human rights groups say that some 700 people have been killed since the unrest began.