DAMASCUS, Syria — A barrage of mortar shells hit government-held areas of the northern city of Idlib on Monday, killing 14 people and wounding at least 50, Syrian state media said.

Idlib is the provincial capital in northwestern Syria and it has been under the control of President Bashar Assad's troops since the beginning of the Syrian conflict in March 2011. Rebels trying to overthrow Assad's government control the areas around the city. They have been besieging the city for more than two years, firing mortars into the government-held areas and clashing with Assad's troops at its outskirts.

The state-run SANA news agency said mortars shells fell on several parts of Idlib on Monday afternoon, including a residential area and a market. State TV said children were among those who died in the attacks, and at least 50 people were wounded.

There was no immediate claim of responsibility for the attacks.

State TV blamed "terrorists" for the attacks, a term the Syrian government uses for rebels trying to overthrow Assad's government.

Also Monday, activists reported heavy clashes between several Syrian rebel factions and an al-Qaida breakaway group fighting for control of a border crossing with Iraq in eastern Syria.

The fighting in the town of Boukamal, on the border between Syria and Iraq, between rebel groups and the Islamic State of Iraq and the Levant comes just hours after the jihadi group declared the establishment of a transnational Islamic caliphate.

The Britain-based Syrian Observatory for Human Rights said the infighting was concentrated in the town and the nearby border crossing as different factions sought control of the frontier.

The al-Qaida breakaway group, which on Sunday declared the establishment of an Islamic caliphate, controls much of northeastern Syria. In Iraq, it has recently captured cities and towns as well as border crossings, effectively erasing the frontier.

The group says its Islamic state stretches from northern Syria to the Iraqi province of Diyala northeast of Baghdad, and has called on all Muslims worldwide to pledge allegiance to it.

Last week, beleaguered fighters of the al-Qaida-linked Nusra Front, which has previously fought the Islamic State of Iraq and the Levant in opposition-held territory in northern and eastern Syria, defected and joined jihadi group in Boukamal —effectively handing over the border town to the powerful group which controls the Iraqi side of the crossing.

Rebel infighting has turned into a war within a war in Syria, three years after the conflict began with largely peaceful protests against Assad, whose family has ruled the country for more than four decades.

After the government brutally cracked down on the protest movement, many Syrians took up arms to fight back. As the uprising shifted into a civil war, the Western-backed Free Syrian Army emerged — a loose term for a collection of self-formed brigades and defectors from Assad's military that fight under a nationalist banner.

But Islamic fighters became the dominant force in the armed opposition, ranging from religious-minded Syrians calling for rule by Shariah law to more extreme al-Qaida-inspired fighters.

The Islamic State of Iraq and the Levant, which was at the time Iraq's al-Qaida branch, barged into the Syrian war in 2012.

Other rebels initially welcomed the jihadis as allies against the Assad government, but soon turned on the group after accusing it of hijacking the uprising for its own transnational goals and imposing a brutal form of Islamic rule in the territories under its control.

Up to 7,000 people, the majority of them militants, have been killed in the rebel-on-rebel violence across the opposition-held territory in the north since January, according to the Observatory's tally, which is compiled by its activists on the ground.

Surk reported from Beirut.