BEIRUT — Syrian opposition fighters battled rival rebels from an al-Qaida-linked faction across parts of northern Syria on Sunday, as deep fissures within the insurgency erupted into some of the most serious and sustained violence between groups opposed to President Bashar Assad since the country’s conflict began.

The clashes, which broke out on Friday and have spread to parts of four provinces, pit an array of moderate and ultraconservative Islamist brigades against the Islamic State of Iraq and the Levant, an extremist group that has become both feared and resented in parts of opposition-held areas for trying to impose its hardline interpretation of Islam.

The fighting did not appear to be a turn in unison by Syrian rebel groups against the Islamic State of Iraq and the Levant, activists and analysts said, but rather an outburst of violence against the al-Qaida-linked group in certain communities where tensions with other opposition factions were already simmering.

FRAGMENTED AND LOCALIZED

In a reflection of the fragmented and localized nature of much of the fighting in Syria’s civil war, the Islamic State of Iraq and the Levant continued to cooperate with rebel factions against government forces in other parts of the country.

But in some corners of opposition-held northern Syria, the backlash against the Islamic State of Iraq and the Levant has been brewing for months. The group, which analysts say boasts more than 5,000 fighters, many of whom are foreigners, elbowed its way into rebel-held areas in the spring, co-opting some weaker armed opposition groups and crushing others as it consolidated its grip on new turf.

That infighting has left scores dead on both sides, and has undermined the broader rebel movement’s efforts to oust Assad. It also has strengthened the government’s position ahead of an international peace conference for Syria expected in just over two weeks.

For the West, meanwhile, the rise of the Islamic State of Iraq and the Levant, as well as another al-Qaida-linked group, the Nusra Front, has been a source of concern, and a major reason that support in Washington and other Western capitals has dwindled in recent months.

BRUTAL TACTICS

Some in northern Syria originally welcomed the Islamic State of Iraq and the Levant for imposing a degree of order on the villages and towns that fell under its control. But the group alienated many by employing tactics deemed brutal even by the standards of Syria’s bloody conflict.

Its fighters have beheaded captured government fighters, and kidnapped anti-Assad activists, journalists and civilians seen as critical of its rule.

The latest and most serious bout of infighting began Friday after residents in the northern province of Aleppo accused members of the Islamic State of Iraq and the Levant of killing doctor Hussein Suleiman.