At least 40 or 50 people with links to Norway have joined militants waging war in Syria and run the risk of returning as seasoned militants, the NIS wrote in its annual threat assessment report on Monday.

"We conclude that the threat has already increased and will continue to increase throughout 2014," the head of NIS General Kjell Grandhagen said, adding that these militants are often in terrorist groups such as the al-Qaeda-linked Islamic State of Iraq and the Levant (ISIL) and al-Nusra Front.

About 2,000 European nationals have left the continent to join terrorist groups fighting in Syria, the NIS has estimated.

Norwegian daily Verdens Gang also reported Monday that about a dozen women have left Norway for Syria to join militants.

In late 2013 the fate of two teenage girls of Somali origin hit the headlines in Norway when they left to join a militant group in Syria and were located weeks later by their father who brought them home.

On January 30, James Clapper, the US director of national intelligence, told the Senate that more than 7,000 foreign militants are fighting in Syria.

Syria sank into war in March 2011 when pro-reform protests turned into a massive insurgency following the intervention of Western and regional states.

The unrest, which took in terrorist groups from across Europe, the Middle East and North Africa, has transpired as one of the bloodiest conflicts in recent history.

RA/SHI