U.S. Secretary of State Hillary Clinton said on Friday that Syria would be better off without Assad, and called on nations that buy oil or sell arms to Syria to cut those ties.

"We urge those countries still buying Syrian oil or gas, those countries still sending Assad weapons, those countries whose political and economic support give him comfort in his brutality, to get on the right side of history," she said.

Syria's oil industry, with which the Assad has close links, generates most of the state's hard currency from crude output of 380,000 barrels per day.

While Syria exports crude oil, its refinery capacity is not sufficient to meet domestic demand for fuel. Trading sources said Swiss oil traders Vitol and Trafigura agreed to supply state firm Sytrol with 60,000 tons of gasoline this week.

The global campaign group Avaaz urged European nations on Friday to impose immediate restrictions on purchases of Syrian oil to "dry up" funding of Assad's security services. It said over 150,000 Avaaz members had signed a petition to that effect.

But there is little prospect of Western states putting teeth into the sanctions on Assad by targeting Syria's oil because of vested commercial interests.

Asked why the United States had not called on Assad to step down, Clinton said Washington had been "very clear" in its statements about Assad's loss of legitimacy, and wanted other nations to add their voices.

On Wednesday Washington imposed sanctions on Syria's largest bank and its biggest mobile telephone company, controlled by Assad's cousin Rami Makhlouf. The next day, U.S. Ambassador to Damascus Robert Ford said more sanctions would follow if the Syrian authorities did not halt the violence.

In addition to the Friday protests, nightly Ramadan prayers, or "tarawih", which follow the breaking of the fast, have given more Syrians a focus for daily protest marches.

In neighbouring Lebanon, hundreds of people demonstrated in the northern, mainly Sunni Muslim, district of Akkar in support of the Syrian protesters.

Regional powers Turkey, Saudi Arabia and Egypt have all applied pressure on Assad to stop the violence.

Turkish President Abdullah Gul warned Assad not to leave reforms until it is too late in a letter delivered to the Syrian president earlier this week, Turkey's state-run Anatolian news agency reported on Friday.

U.S. President Barack Obama and Turkish Prime Minister Tayyip Erdogan spoke on Thursday and agreed Syrians' demands for a transition to democracy must be met, the White House said.