Spanish police on Tuesday (June 18th) detained 10 nationals of Syrian origin accused of financing "al-Qaeda terrorist militia" in Syria, the interior ministry and a source close to the probe said.

With Europol's collaboration, more than 350 agents took part in an operation in Madrid, further south in Toledo and the eastern province of Valencia, AFP reported.

Fourteen homes and other premises were searched by officers.

The source, who refused to be named, said the detainees were Spanish of Syrian origin.

"The organisation was led by a family clan that for years allegedly used a legal business structure to hide illicit operations with which they evaded tax authorities and laundered large amounts of money," the ministry said in a statement.

"The detainees allegedly took away money from each legal operation, substituting the real amount of the operation in the receipt with another lower value amount," it said.

A large part of the stash of cash amassed went to the opposition-held province of Idlib in Syria to "give support and financial backing to terrorist militia there".

Relatives of the suspects are allegedly al-Qaeda members.

The money was sent via various methods including human couriers and the "hawala" system, an informal method of payment based on trust that is far more difficult to trace than bank transfers.

One of those detained had already been arrested in Syria in 2008 and sentenced for his participation in an attack and for belonging to extremist group Fatah al-Islam.

The operation comes just weeks after a Syrian man was detained on suspicion of funnelling money to European extremists linked to the "Islamic State of Iraq and Syria" (ISIS) so that they could return to Europe.