An attack on a meeting of a Syrian rebel chiefs in the north of the war-ravaged country Tuesday killed at least 28 leaders from one group, a monitor said.
"Twenty-eight heads of the Ahrar al-Sham group were killed in an explosion that targeted a meeting tonight... in Idlib province," Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP.
The death toll was expected to mount as around 50 military and religious leaders attended the meeting in the basement of a house at Ram Hamdan, northeast of Idlib city.
Among those who died was Ahrar al-Sham leader Hassan Abbud, said the Islamic Front, the country's biggest rebel alliance, in a statement on Twitter.
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Neither the Islamic Front and Abdel Rahman were able to say who may have been behind the attack.
Ahrar al-Sham is the main group in the Islamic Front, which has been battling to oust Syrian President Bashar al-Assad.
In recent months it has been locked in fighting with the Islamic State (IS), a jihadist group that has seized swathes of territory in Syria and neighbouring Iraq.
Nearly all of Idlib province is under the control of various rebel groups, including the Islamic Front. Its capital city, also called Idlib, remains in the hands of the regime.
The Syrian conflict has killed more than 191,000 people since March 2011.