BERLIN (REUTERS) - Germany faces an increased threat of attack due to around a doen German militants who have returned from the conflict in Syria with specialist knowledge of weapons and bomb-making, German intelligence officers said.

Hans-Georg Maassen, head of Germany's domestic intelligence agency (BfV), said about 300 German citizens had left to join rebels fighting President Bashar al-Assad in Syria since the conflict began in 2011, and more than 20 had died there.

"We have knowledge of about a dozen people who were active in the conflict in Syria ... and with this the threat of a terror attack in Germany increases," Mr Maassen said.

About 10 per cent of those who have travelled to Syria are Muslim converts and about 7 per cent women, the agency said.

It said some 20 to 30 individuals had returned to Germany from the Middle East and were under surveillance. A dozen of these are deemed dangerous.

"I wouldn't put it past them to plan attacks," said a second official, who requested anonymity.

There were signs that veterans of the conflict returning to other European countries had started to plan attacks but had been thwarted, according to the BfV.

About 2,000 people are believed to have left European states to head to Syria. Most join the Nusra Front and the Islamic State of Iraq and the Levant (ISIL) - the two militant opposition groups that are closest to al Qaeda and considered most dangerous by the West.

For many Europeans, the trip is as simple as a cheap flight to Turkey and short ride over the border with a trafficker.

Turkish Foreign Minister Ahmet Davutoglu told Reuters in an interview published on Wednesday that Syria was becoming a risk for all European countries because of the number of fighters who had travelled there.

He called for international cooperation to stop the flow of foreign fighters into Syria and denied suggestions that Turkey, which is hosting more than 700,000 Syrian refugees, was letting foreign fighters cross its porous border into Syria.

Britain this week arrested four people on suspected terrorism offences related to Syria, including a former Guantanamo Bay prisoner.

British police have already arrested 16 people on suspicion of such offences this year, some as young as 17, compared to 24 such arrests in all of 2013.