The concern within British and U.S. intelligence agencies is the Syrian civil war, now entering its third year, will breed a new generation of jihadist that include a large number of western recruits among its ranks.
The Pentagon and State Department have assessed the number of extremists within the rebel forces remains at less than 20 percent.
But senior U.S. military officials say the number of hardcore Islamist groups stands at over 50 percent and is steadily increasing as the war drags on.
Defense Secretary Chuck HagelChuck HagelLobbying World The US just attacked Syria. So where's Congress? Senators tear into Marines on nude photo scandal MORE and Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey have opposed a White House plan to provide U.S. arms to Syria's rebels, over concerns of growing extremist influence in the country.
But Washington is pressing ahead with that plan, directly supplying weapons and equipment to the rebels.
The weapon supplies, consisting mostly of small arms, ammunition and anti-tank weapons, are being coordinated by the CIA and limited to vetted portions of anti-government forces in Syria, such as the Free Syria Army.
President Obama approved the weapons program back in July, giving the CIA the green light to begin arming Syrian rebels from clandestine bases in Turkey and Jordan, according to reports at the time.
But those deliveries come as violent clashes between rebel forces and extremist groups have begun to boil over in Syria.
Turkish fighters shot down a Syrian military helicopter along the country's border with Syria in September, in response to sporadic firefights along the border region.
Jabhat al Nusra gunmen exchanged fire with Kurdish fighters from the Democratic Union Party, a militant separatist group based in Turkey, sparking the shoot down.
The fighters were conducting aerial surveillance missions over the town of Ras al-Ain on the Turkey-Syria border where the most intense fighting took place, according to reports at the time.