Damascus, Dec 18 (IANS): Islamic State (IS) militants killed 20 rebels of the Free Syrian Army (FSA) overnight near Damascus, media reported Thursday.

The IS executed the FSA fighters after capturing them in the eastern part of the al-Qalamoun region in the northern countryside of Damascus, Xinhua reported citing the pan-Arab al-Mayadeen TV.

The militants have also threatened to kill three other FSA leaders in the same area if they did not pledge allegiance to the leader of the IS, Abu Bakr al-Baghdadi.

The FSA is a Western-backed opposition grouping, that has been considered as "moderate" and could confront government troops and radical groups whose presence has increased in Syria.

Syrian government troops have been engaged in battles against rebel groups and extremists since last year in the western part of al-Qalamoun, a rugged region between the central province of Homs and Damascus.

The Syrian troops managed to sever the rebels' main supply line from neighbouring Lebanon during the battles that lasted from last year's winter till the spring of 2014.

However, an array of militant groups are reportedly assembling in the eastern part of al-Qalamoun to fight both government troops and the IS.

Over 190,000 people have been killed and millions of others displaced inside and outside the country as a result of the Syrian crisis, which has become more complicated when radical groups capitalised on the chaotic situation to achieve territorial gains.