The Observatory Tuesday said 6,300 fighters had joined the training camps of that Al Qaeda breakaway group in Syria's northern provinces of al-Raqqa and Aleppo, Xinhua reported.
Among them, 5,000 recruits are Syrians, the Observatory said, adding that the rest are newly-recruited fighters.
The Observatory, which relies on a network of activists on the ground, said that 1,300 fighters are Arab and foreign nationals, adding that a majority of them entered Syria from Turkey.
It said that the IS pays a single Syrian fighter 400 dollars a month. In case of married militants, their kids and spouses also get paid. As for fighters from overseas, they have a monthly pay of 800 hundred dollars.
The IS, previously known as the Islamic State in Iraq and the Levant (ISIL), has recently proclaimed it established an " Islamic Caliphate" in parts of Syria and Iraq, threatening the political geography of the region.