The monitor group said almost all of those killed were “terrorists” from ISIS and Al-Qaeda's Syrian affiliate Al-Nusra Front. The Britain-based monitor said the strikes that began on September 23 had killed 1,465 members of ISIL, most of them non-Syrians, Daily STARS reports.

Another 73 fighters from Al-Nusra Front were killed, along with a man from a terrorist group being held prisoner by ISIL in the group's de facto capital Raqa.

Meanwhile a senior U.S. military figure has claimed that airstrikes against ISIL have killed 8,500 extremist fighters.

U.S. Central Command chief, General Lloyd Austin, was speaking while meeting Iraqi Defense Minister Khaled al-Obeidi on Monday.

Austin said that the "Islamic State of Iraq and the Levant" militants were killed during U.S.-led coalition airstrikes in Iraq and Syria, in comments reported by the Iraqi defense ministry, ANADOLU AGENCY reported.

The U.S. and partner nations have carried out more than 1,300 air strikes against ISIL in Iraq and Syria since operations began in August, according to the force.

Washington and a small coalition of Arab countries began strikes against ISIS in Syria last year, expanding US-led operations with a broader coalition already underway against ISIS in Iraq.    

ISIL emerged in Syria in 2013, growing from Al-Qaeda's former Iraqi affiliate.    

But it broke with Al-Qaeda and declared an Islamic "caliphate" in territory it controls in Syria and Iraq, attracting a steady stream of foreign fighters and carrying out abuses including beheadings.