The Philippines has detained 18 Chinese men on suspicion of illegal black sand mining in the northern coastal town of Aparri, the justice department said Tuesday.

Authorities say there has been a rise in the illegal extraction of magnetite -- also known as black sand -- which is an iron ore in huge demand by China's steel mills.

Justice department investigators raided two mine sites run by Chinese firm Hua Xia Mining and Trading Corp. last Thursday and detained 18 of its employees, department spokesman Alex Lactao told AFP.

The company had a permit to dredge magnetite from a nearby river but not from the coast, he said. It is illegal to extract any minerals within 200 metres (656 feet) of a beach under Philippine law.

"Nine Chinese nationals were burrowing and processing magnetite sand within the prohibited zone," said Lactao.

The other nine were arrested at a nearby beach where they were building a magnetite processing plant, he added.

Environmental groups say illegal magnetite mining has been stripping Philippine coasts through erosion.

They have blamed small-scale mining firms, most of them allegedly Chinese and often operating in collusion with shady local government officials, for the devastation.

The men detained in Aparri lacked permits required to work in the Philippines and could face further criminal charges, said Lactao, adding they have now been handed over to the immigration bureau.

The justice department launched a third raid in the northern town of San Vicente on Saturday but found the site, previously operated by another Chinese outfit, had been abandoned, the official said.

The raid teams seized mining equipment from all three sites as well as 1,500 tonnes of magnetite concentrate from the Aparri operations, he added.

Chinese embassy spokesmen could not immediately be reached for comment on Tuesday.

Under the Philippines's mining laws, the environment ministry has regulatory oversight over large operations but not small-scale miners, who are defined as using only light equipment and no explosives.

Instead small-scale miners are licensed by local governments, which often lack the expertise or will to properly supervise them.

Black-sand miners are increasingly flouting the law by mining near the nation's beaches, Carlos Tayag, Mines and Geosciences Bureau director for the northwestern Philippines, told AFP in May.

Reports of Chinese nationals breaking these laws have occurred frequently in recent years.

Authorities arrested 80 Chinese miners from one chromite mine in Zambales in 2010, and another eight at a similar chromite operation in the central island of Samar last year.