BEIRUT — Fierce clashes between Syrian rebels and al-Qaida-linked militants spread through northwest Syria on Wednesday, as the two sides jostled for control over an important border town.

At least 11 people, among them 3 civilians, were killed, according to the Britain-based Syrian Observatory for Human Rights. The toll could climb as the rivals switch to heavier weaponry such as tanks, artillery, and suicide bombings.

Local media activist Ahmad Abazeid said a powerful and conservative rebel faction called Ahrar al-Sham had consolidated control over Sarmada after seizing the offices of the al-Qaida-linked Levant Liberation Committee. Sarmada controls trade through the vital Bab al-Hawa crossing with Turkey.

But the Levant Liberation Committee seized the next town over, al-Dana, according to the extremists’ own Abaa’ News Agency, meaning they could still control the movement of goods through Idlib, the opposition-held province in northwestern Syria that is home to close to a million Syrians displaced by fighting.

The infighting comes days after Ahrar al-Sham adopted the tricolor Syrian rebel flag beside its own white jihadist flag as it endeavors to win over mainstream support in opposition-held parts of Syria.

It was “another nod toward moderation,” said Syria analyst Sam Heller, after the group pledged to adopt an internationalized judicial code for its courts. Factions in rebel Syria often compete with one another to run the courts in their respective zones of influence.

But the shift has alienated the al-Qaida-linked group, which dominates Idlib province and often pulls down displays of the green, white, and black tricolor in favor of its own black flag. It also abides by a harsh judicial code.

The jihadis fired on demonstrators waving the tricolor flag in province’s capital, also called Idlib, on Tuesday, sparking the clashes, said the Observatory.

The Observatory also reported fighting around Saraqib, Salqin and other towns dotting Idlib province.

Rebels and al-Qaida often fight during lulls in the Russian and Syrian government air campaigns that have battered the province since 2013. But the skies have been quiet since May, when the government agreed to abide by a “de-escalation zone” that encompassed Idlib.

“When there is a cease-fire and the fronts calm down, these factions tend to turn inwards,” said Heller.

Meanwhile Wednesday, at least 30 civilians were killed in airstrikes on two separate villages held by the Islamic State group, according to opposition activists.

The monitoring group Deir Ezzor 24 said a Russian airstrike killed 15 people, including an entire family, in the eastern Syrian village of Ayash. The Observatory said a U.S.-led coalition airstrike killed another 15 people in the village of Zour Shimr, near Raqqa, the de facto capital of the IS group. The Observatory also reported the strike on Ayash.

The Associated Press could not independently identify the aircraft involved.

The U.S. military is supporting local Syrian forces in a campaign to drive IS from Raqqa.