Police said the investigation began after the dismantling of a migrant-trafficking gang transporting mostly Syrians between Algeria and Spain, which led to a probe into their finances.

An Arab branch of the network "took charge of the reception of money in any part of the world", while a separate Chinese branch supplied the cash in Spain in exchange for cryptocurrencies.

Police arrested 17 mostly Chinese and Syrian suspects in January -- 15 in Spain, one in Austria and another in Belgium -- said EU law enforcement agency Europol which supported the operation.

The network's Belgium-based leader had "Jordanian-Palestinian nationality" and facilitated contacts within Spain, police chief inspector Encarna Ortega told a press conference in Madrid.

He is suspected of coordinating a litany of operations, mainly laundering money from the proceeds of trafficking humans and drugs, she added.

In total, the suspects moved $21 million between June 2022 and September 2024, Spanish police said.

Authorities seized from them 205,000 euros ($229,000) in cash, more than 183,000 euros in cryptocurrency, 18 vehicles, real estate property and illegal cigars worth more than 600,000 euros destined for sale in China.

Hawala is traditional system of moving money between countries based on confidence and a network of intermediaries with minimal paperwork which is popular in parts of Asia and Africa.

The method is especially common among migrant workers who send remittances to their families, but it has also been linked with financing terrorism.