DAMASCUS, Sept. 26 -- A total of 235 fighters in Syria have joined the Islamic State (IS) militant group since U.S. President Barack Obama declared war against the IS on Sept. 10, the Syrian Observatory for Human Rights was cited by Lebanon's al- Jadeed TV as saying Friday.

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.

The Observatory said the majority of the new recruits were Syrians from the al-Qaida-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 U.S. administration has recently felt the threat of the radical group, lobbying international support for striking the IS in Iraq and Syria.

The U.S.-led anti-terror coalition started its strikes against the IS in Syria last Tuesday, killing tens of the extremist group' s members so far. The strikes have been focusing on the IS strongholds in the northwestern province in al-Raqqa and the eastern province of Deir al-Zour.

Syria responded by announcing that it was informed ahead of the attacks, adding that the administration of President Bashar al- Assad stood by any international effort to eradicate the " terrorist groups" from the country.