Beirut ? 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 U.N. 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.

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.

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 percent 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 European Union.