ROME (AFP) - Italian police said on Thursday they had arrested the crew of a ship carrying more than 200 refugees believed to be from Syria, boarding and detaining the vessel in international waters in the first operation of its kind.

"This is the first capture carried out in international waters to implement international conventions... to detain and inspect a ship not flying a national flag," the police said in a statement.

The 30m long vessel was referred to by police as a "mother ship" used by people smugglers to bring migrants to Italian waters then put them on smaller vessels to reach the coastline.

It was spotted 200km south of Sicily by a Romanian patrol boat taking part in the European Union's Frontex border guard service.

Italian police have arrested all 15 crew members and detained the ship itself.

The migrants included 85 men, 50 women and 84 minors, and all said they were refugees from Syria.