Syrian rebels laid siege to jihadists from the Islamic State of Iraq and the Levant in their northern stronghold Monday, hoping to crush the Al-Qaeda affiliate accused of widespread abuses.
A broad coalition of moderates and Islamists opposed to President Bashar al-Assad is seeking to drive ISIL -- which is accused of kidnapping, torturing and killing rival rebels and civilians -- from its stronghold in the northern city of Raqa.
The new front in Syria's increasingly complex civil war opened less than three weeks away from a planned peace conference, for which the United Nations has started sending out invitations, excluding Assad's ally Iran.
The Syrian Observatory for Human Rights said rebels in Raqa managed to free 50 Syrian prisoners held by the Sunni extremists, who are believed to be holding hundreds of prisoners, including foreign journalists.
Raqa is the only provincial capital lost by the regime since the conflict began in March 2011 with peaceful protests demanding democratic reform but escalated into a full-blown war when Assad's forces launched a brutal crackdown on dissent.
The city later fell into the hands of ISIL, the latest incarnation of Al-Qaeda's Iraq affiliate, which joined the fight against the regime in late spring 2013.
The rebels initially welcomed the battle-hardened jihadists, but tensions mounted as ISIL was accused of imposing a reign of terror in areas where it operates, especially Raqa.
Three powerful rebel alliances on Friday launched what activists called a second "revolution," and have advanced quickly, expelling ISIL from checkpoints and bases across Aleppo, Idlib and Hama provinces.
The Observatory said the main group besieging ISIL's Raqa headquarters is Al-Nusra Front, which is also affiliated with Al-Qaeda but is seen as less extreme and has long competed with ISIL to represent the global terror network in Syria.
ISIL has struck back, including with a car bombing at a rebel checkpoint in Darkush, Idlib that killed an unknown number of fighters on Monday, the Britain-based Observatory said.
In Aleppo, a 16-year-old ISIL suicide attacker detonated himself at a rebel checkpoint, killing one opposition fighter and wounding several more.
A key complaint among the rebels fighting ISIL is that the self-styled Islamic state sought hegemony over areas under its control, while activists and rights groups accused it of torturing and killing its rivals, including with public executions.
On Monday, the body of a decapitated child was found near ISIL's headquarters in Kafranbel, Idlib, the Observatory said.
The rebel side also committed abuses, executing two ISIL fighters and burning one of their bodies in the Haydariyeh district of Aleppo, "in front of neighbourhood residents", the group added.
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While many in the opposition welcomed the war on ISIL, some fear it threatens to distract from the battle with Assad's forces, which have continued to bombard rebel-held areas.
In rebel-held Bazaa in Aleppo province, an air raid killed 12 people including three children, the Observatory said.
It was the latest in a regime air campaign against Aleppo that killed some 550 people between December 15 and January 2, according to the monitoring group.
The Syrian conflict is estimated to have claimed more than 130,000 lives, and has forced millions more to flee their homes.
A security official in Damascus told AFP the fighting between the rebels and ISIL was "expected, and will become more violent".
UN leader Ban Ki-moon, meanwhile, started sending out invitations to the so-called Geneva 2 peace talks, but Assad's key ally Iran was not on the first list, UN deputy spokesman Farhan Haq said.
Iran brushed off the move, saying it would only accept an offer to participate that respected its "honour".
The decision may help pave the way for the opposition National Coalition, which has repeatedly stipulated Iran must not be invited, to participate.
The Coalition, which re-elected Ahmad Jarba as its leader on Sunday, will discuss Tuesday whether to attend the talks, although a key group -- the Syrian National Council -- has already announced its boycott.
The 30 countries invited to the Geneva talks include Saudi Arabia, a major backer of the Syrian opposition, as well as Britain, China, France, Russia and the United States -- the five UN Security Council permanent members -- and Syria's neighbours such as Turkey, Iraq and Jordan.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov will meet on January 13 in a bid to decide Iran's role in ending the nearly three-year-old war, said UN deputy spokesman Farhan Haq.
Tehran's role has been one of many obstacles that have bedeviled efforts by Ban and UN-Arab League peace envoy Lakhdar Brahimi to organize the conference.
The makeup of the Syrian opposition and government delegations has also called friction.