Rebel-on-rebel clashes have killed nearly 700 people over the past nine days in northern Syria in the worst bout of infighting among the opponents of President Bashar Assad since the country's civil war began, activists said Sunday.

The fighting between the al-Qaida-linked Islamic State of Iraq and the Levant and several Islamist and more moderate rebel brigades has broken out in cities, towns and villages of at least four opposition-held provinces in the north. It has added yet another layer of complexity to Syria's nearly 3-year conflict, while also overshadowing the broader battle against the government over the past week.

The British-based Syrian Observatory for Human Rights activist group said Sunday that at least 697 people have been killed since the rebel infighting began Jan. 3. The toll includes 351 fighters from the Islamist and mainstream opposition brigades, 246 from the "Islamic State," and 100 civilians.

The "Islamic State" muscled its way into opposition-held territory in northern Syria last spring, coopting some weaker rebel brigades and crushing others. Initially welcomed by some residents for bringing a measure of order, the extremist group over time alienated many other rebel factions and large chunks of the civilian population by using brutal tactics to implement its strict interpretation of Islamic law. It has also kidnapped and killed its opponents.

The rebel infighting comes less than two weeks ahead of a planned international conference in Switzerland that aims to broker a political solution to the Syrian civil war. But the prospects for success at the peace talks appear slim at best, and it remains unclear whether they will indeed take place.

Assad has said he will not hand over power, while the Western-backed opposition in exile, known as the Syrian National Coalition, is in disarray and not yet decided whether it will attend the gathering. Even if it does, it is in no position to wrest concessions from Assad, whose forces have seized the momentum in recent months.

At the start of two days of meetings Sunday in Paris, top envoys from 11 countries that support the Coalition were pressuring the group to attend the peace conference, saying the talks were the only way to end the carnage.

In Damascus, meanwhile, Assad made a rare public appearance, attending prayers at the al-Hamad Mosque to celebrate the birthday of the Prophet Muhammad. Syrian state TV broadcast video of Assad shaking hands with a crowd of admirers as he walked through the mosque.

While the rebel war-within-a-war has grabbed the spotlight over the past nine days, the fight against the government has raged on across the country.

In the central city of Homs, government shelling killed more than 20 people Saturday in the rebel-held Waer neighborhood, the Observatory said. It warned that the death toll could rise because dozens of people were critically wounded in the bombardment.

Another activist group, the Local Coordination Committees, also reported the shelling in Waer.

Syrian rebels also have targeted towns and neighborhoods loyal to Assad with indiscriminate mortar fire. On Sunday, Syria's state media said rebel-fired mortar shells killed two people in the pro-government Zahra area of Homs.

The Syrian conflict began in March 2011 with largely peaceful protests against Assad before shifting into an armed insurgency after a brutal government crackdown. It has killed more than 120,000 people, forced more than 2 million to flee the country and devastated the nation's cities, economy and social fabric.

In Damascus, United Nations humanitarian chief Valerie Amos called on the international community to do more to help Syrians suffering from the conflict.

"The world must do more for all the people who are displaced," Amos said. "Many families are living in abandoned buildings, schools or in makeshift shelters, without enough food, clean water or medicine. We must help them to get through this very cold winter."

Amos was in Damascus for talks with government officials ahead of a donors' conference in Kuwait set to open Wednesday for Syrian humanitarian relief.