A total of 162 fighters joined the IS in the northern province of Aleppo in the week ensuing Obama's Sept 10 speech, said the Observatory, adding that 73 others joined the terror group on the 23rd and 24th of this month, Xinhua reported.
The Observatory said the majority of the new recruits were Syrians from the Al Qaeda-linked Nusra Front, except for 15 foreigners who were among the new recruits.
The UK-based Observatory said earlier that 6,300 fighters joined the IS in July alone.
The IS, previously known as the Islamic State in Iraq and the Levant (ISIL), has emerged as a striking radical power among the various rebel groups in Syria. The groups' sway stretches between Iraq and Syria.
The US administration has recently felt the threat of the radical group, lobbying international support for striking the IS in Iraq and Syria.