About one thousand foreign fighters are believed to join the twin conflicts in Syria and Iraq every month amid continuous airstrikes and stricter measures preventing the migration of militants into the embattled regions, U.S. intelligence and the new United Nations report revealed earlier in the week.

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"Numbers since 2010 are now many times the size of the cumulative numbers of foreign terrorist fighters between 1990 and 2010-and are growing," the report obtained by the Associated Press revealed.

Experts believe that at least 16,000 foreign fighters have already joined the Islamist groups in Syria, an unprecedented scale far exceeding the largest migration of fighters during the 1980s Afghanistan war.

An unnamed U.S. intelligence official further warned of rising numbers among foreign fighters, The Washington Post reported.

This, in spite of the continued offensive strikes against Islamic State positions in Syria and Iraq which have reportedly killed 460 members, while leaving about 60 dead from the ranks of the Nusra Front, a Syrian al-Qaeda affiliate.   

The U.S.-led campaign against the Islamic State, also previously known as ISIS and ISIL, has so far mounted 600 strikes in both Syria and Iraq.

The anti-ISIS coalition has mostly dropped bombs to halt the group's bloody advance while giving Iraqi and Syrian troops-the main combat force on the ground-time to regroup.

Pentagon spokesman, Rear Adm. John Kirby said this week that the strikes have disrupted the militants' operation.

Since the air campaign began earlier this year, military analysts have been tracking how the assaults would affect would-be jihadists.

Counterterrorism expert, Andrew Liepman, said given the swelling number of migrating militants, the airstrikes have neither discouraged nor served as the rallying cry for potential fighters.      

The panel of experts who have drafted the report said the fighters stream from 80 countries, including France, UK, Russia, and countries in Africa and the Middle East. Experts are concerned that these militants carry passports that would allow them to travel back to their countries to launch domestic terrorism.

TagsISIS foreign fighters, Syrian airstrikes