AMSTERDAM (REUTERS) - Dutch cities need more powers to tackle battle-hardened militants returning home from the Middle East, a mayor said on Thursday, a day after authorities announced the arrest of 12 people with suspected links to militant Islamists.

Thousands of Western volunteers have travelled to Syria and Iraq to join militant groups such as Islamic State, triggering fears among European governments and security forces that returning fighters may carry out attacks on their home turf.

The mayor of the Hague, seat of the Dutch government and home to a host of international organisations, said Dutch municipalities lacked the administrative capacity and the legal resources to deal with the threat of returning militants.

"Returning or detained jihadis must immediately be screened," Jozias van Aartsen said in a statement which called for the power to put them into compulsory de-radicalisation programmes.

"Where necessary, it should be possible to force treatment upon them," he said, adding that existing de-radicalisation programmes were only voluntary.

The Hague has some of the Netherlands' most economically deprived and ethnically diverse neighbourhoods and has come to be known as a hotbed of radical Islamist activity.

Dutch national anti-terrorism coordinator Dick Schoof said in a report published on Wednesday that police had arrested 12 people in the Hague for suspected militant Islamist activity.

He also said Dutch authorities were currently conducting about 30 national criminal investigations involving some 60 people suspected of involvement in recruiting fighters for conflicts in the Middle East.

"All identified jihadis are immediately interrogated by the police or the gendarmes on their return," he said.

"The first criminal cases against returned jihadis have begun."

About 5 per cent of the Dutch population is of Muslim heritage, many of them originally from Morocco and Turkey, but many of them are concentrated in certain parts of major cities.

In June, Dutch authorities estimated that 120 Dutch citizens had gone to fight in Syria's civil war, now into its fourth year, and that about 14 of them had been killed in combat.

Other European countries face similar challenges.

Earlier this week, German police arrested nine men suspected of supporting militant groups in Syria and raided numerous properties in one of the largest sweeps against alleged Islamists yet in Germany.

Police in Bosnia arrested 11 people on Thursday on suspicion of fighting alongside Islamist militants in Syria and Iraq.