With the Syrian conflict entering its fourth year, governments across Europe are growing increasingly concerned about the number of their nationals heading to Syria to fight.

Interior Minister Ronald Plasterk, speaking at the launch of the Dutch intelligence agency's annual report, said one Dutch national had travelled from Syria to Iraq before carrying out an attack with a bomb in a rucksack. The other carried out a car bomb attack in Syria.

Plasterk declined to identify the attackers or give details of casualties in the suicide blasts.

The number of Dutch citizens leaving for Syria to fight on the side of foreign-backed militants battling to overthrow President Bashar al-Assad had grown sharply in 2013, Plasterk wrote in the introduction to the report.

"The consequence is that at the end of 2013, dozens of Dutch jihadists have become habituated to extreme violence and have become radicalized in their intolerant and violent ideological orientation," the agency said.

The General Information and Security Service said about 100 Dutch citizens had travelled to Syria in 2013, joining the Foreign-backed militant groups Islamic State in Iraq and the Levant (ISIL) and al-Nusra Front. About 20 had returned.

On Tuesday, France unveiled policies aimed at stopping its citizens joining the Syrian civil war and preventing young French Muslims becoming radicalized and posing a threat to their home country.

French nationals who return from Syria could automatically face charges of being part of a terrorist organization, while minors would be prevented from leaving France without parental consent.

Syria has been gripped by deadly unrest since 2011. According to reports, Western powers and their regional allies -- especially Qatar, Saudi Arabia, and Turkey -- are supporting militants operating inside the country.

More than 150,000 people have reportedly been killed and millions displaced so far due to the turmoil in the country.

HH/HH