The European Union banned oil imports from Syria on Friday in a move that will cost the embattled regime millions of dollars each day as it tries to crush a resilient uprising that has railed against the country's authoritarian government for more than five months.

President Bashar Assad's assault on protests continued, killing at least 13 people, activists said. Security forces fired on thousands of anti-government protesters and surrounded mosques to prevent worshippers from streaming into the streets to join the rallies.

The UN estimates some 2,200 people have been killed since March as protesters take to the streets every week, despite the near-certainty that they will face a barrage of bullets and sniper fire. But the regime is in no imminent danger of collapse, leading to concerns violence will escalate.

A large Syrian flag with the words 'Freedom, Syria' written on it in Arabic is carried in the streets of Idlib, Syria, by protesters. Because access to Syria is restricted, The Associated Press could not independently verify the content of this photo. SHAMSNN/Associated Press

On Friday, Syrian protesters marched under the slogan "Death Rather Than Humiliation" — reflecting a growing frustration among activists that their largely peaceful gatherings have failed to crack the regime.

Recent weeks have seen a subtle change in tone among some protesters who are calling on Syrians to take up arms and inviting foreign military action like the intervention that helped topple the Libyan government.

Although the calls to arms are not widespread, there are fears that a prolonged stalemate could serve to radicalize the movement and lead to more bloodshed.

Majority of Syrian oil goes to EU

The EU oil ban follows other international sanctions and worldwide reproach, although Assad has brushed off the condemnation as foreign meddling. The oil embargo is significant because Damascus gets about 28 per cent of its revenue from the oil trade and sells fuel to France, Germany, Italy and the Netherlands.

Syria exports some 150,000 barrels of oil per day, with the vast majority going to the EU.

European Union trade spokesman, John Clancy, said Syria earned €3.1 billion euros ($4.3 billion) by selling oil to the EU in 2010. Oil from Syria amounted to 1.5 per cent of EU's total crude oil imports that year.

"The impact of the ban in terms of EU oil supply is very minimal, but the impact on the financing of the Syrian regime is quite substantial," Clancy said.

Without that revenue, Syria will likely burn far more quickly through the $16.6 billion in foreign reserves it had at the start of the uprising. Still, some analysts believe Syria is getting financial assistance from Iran, which would cushion the EU blow.

"There is little evidence to suggest that sanctions will be enough to end the violence and force sincere political reform," according to a briefing this week by Maplecroft, a British-based risk analysis company.

"Although the EU embargo could hit the regime hard ... Assad will continue to rely on financial support from Iran," the briefing said.

Asia also could present a new market for the oil as China has close ties to Damascus.

Syrian regime is 'unacceptable'

The EU ban covers the purchase, import and transport of oil and other petroleum products from Syria. The EU also has banned European banks from opening credit lines for such sales, and prohibited insurance companies from insuring the cargoes.

In addition to the oil ban, four more Syrian individuals and three entities were added to a list of those facing an EU asset freeze and travel ban.

Over the past few months, the EU has imposed travel bans and asset freezes against 35 Syrian government officials and military and police commanders, including Assad himself. Other countries, including the U.S., also have imposed sanctions but Syria's business with those nations was limited.

The EU has in the past been reluctant to ban Syrian oil and gas imports for fear of the impact on the Syrian public and small businesses.

Explaining the shift, French Foreign Minister Alain Juppe explained that EU nations were hoping the oil ban would send an unmistakable message that "the behaviour of the Syrian regime has been unacceptable."

"We cannot accept such brutal repression against the aspirations of the people," he told the Associated Press during a visit to Poland.

The EU oil embargo will bring the 27-nation bloc in line with the latest U.S. moves to isolate Assad's regime, including a ban on the import of petroleum or related products.

The embargo takes effect on Saturday, but existing contracts can be fulfilled until Nov. 15.