Syrian dissidents on Tuesday urged US President Barack Obama to call on Syrian leader Bashar al-Assad to quit power and pressed for UN sanctions over the regime's deadly crackdown on protests.

The demands -- presented in a first meeting between US Secretary of State Hillary Clinton and members of Syria's disparate opposition -- added to US domestic pressure on the White House to take a tougher line against Assad.

Radwan Ziadeh told reporters he and other US-based dissidents told Clinton in more than one hour of talks that they need "President Obama to address the Syrian people and ask President Bashar al-Assad to step down immediately."

Mohammad Alabdalla, another Washington-based Syrian opposition activist, said such a high-profile US call for Assad to quit power would bring more protesters into Syria's streets.

Clinton said Washington is "working to move forward with additional targeted sanctions" and exploring broader sanctions that would "isolate the Assad regime politically and deny it revenue with which to sustain its brutality."

"The United States will continue to support the Syrian people in their efforts to begin a peaceful and orderly transition to democracy," Clinton said in a statement after the meeting.

Clinton said last month that Assad had "lost legitimacy" after his loyalists attacked the US and French embassies over alleged meddling in Syrian affairs, but US officials have yet to call on him to step down.

When asked why Washington was reluctant to call on the Syrian leader to quit power, Ziadeh said administration officials feared the Assad regime would try to fan sectarian flames and spark a civil war.

But "we addressed this concern" and showed that different groups, including Christians, were involved in the protest movement, he said.

Robert Ford, the US ambassador to Syria who was back in Washington for his Senate confirmation hearing, told senators he attended Clinton's talks with the opposition, calling it a diverse group that "is not very well organized."

He told the Senate Foreign Relations Committee that it is important that the opposition develop Syrian rather than American ideas about how the democratic transition should proceed.

But Ziadeh insisted Washington had an important role to play, and should lead Western efforts at the UN Security Council to impose more sanctions on top of existing US and EU restrictions on Assad and other regime members.

He also called for US pressure "to refer the crimes against humanity committed in Syria to the International Criminal Court," which has issued an arrest warrant for Moamer Kadhafi for his bloody crackdown in Libya.

The 15-nation Security Council remains divided over how to react to the Syria bloodshed, with Western nations demanding tough action and both China and Russia threatening to veto any formal resolution.

Clinton made a veiled appeal on Monday to Russia and China to condemn Syria at the Security Council, which met again Tuesday for a second day of consultations about the worsening violence.

Ford said those UN members who had opposed condemning Damascus are "potentially now more open to some kind of action" following a bloody Sunday in which some 140 people were killed in Syria, including more than 100 in the flashpoint city of Hama.

Meanwhile, US Senators Kirsten Gillibrand, a New York Democrat who is close to Clinton, Mark Kirk, a Republican, and Joseph Lieberman, an independent, pressed for tougher US action against Syria.

They said in a statement they will introduce "new bipartisan legislation that would establish tougher US sanctions against Syria and hold President Bashar al-Assad's regime accountable for its human rights abuses."

The bill would call on Obama to "block access to the US financial system, markets, and federal contracts for companies that invest in Syria's energy sector, purchase the country?s oil, and sell gasoline to Syria," they said.

Such sanctions would bite because about one-third of Syria's export revenue comes from oil, they added.

Ford said the protesters he met during a visit to Hama last month that prompted Syrian anger -- protesters he described as peaceful and "not anti-American at all" -- were committed to bringing about democratic change.

"They are not going to stop," the ambassador told senators.

Ford arrived in Syria in January, almost six years after Washington withdrew ambassador Margaret Scobey following the February 2005 assassination in Beirut of ex-premier Rafiq Hariri in a massive car bomb.