"Op Naga", carried out from May 2-16, wasled by the Malaysian Maritime Enforcement Agency and involved joint enforcement and patrolling efforts in the waters off and airspace over Pahang, Terengganu and Kelantan, The Star Online reported. 

This is the toughest move yet by the Malaysia to curb encroachment by foreign fishing vessels, especially Vietnamese, into its waters.

Authorities checked 266 foreign fishing vessels in all and 25 of them have been seized. 

The Malaysian Home Ministry said on Saturday that the detained Vietnamese fishermen were being investigated under the Fisheries Act for fishing without the permission of the Fisheries director general and violating the Immigration Act for entering Malaysia without proper documentation.

"Op Naga will be continued to ensure that there is no trespassing in Malaysian waters and that local fishermen can carry out their activities without disturbance and competition from foreign fishermen."

Vietnamese fishermen are often charged with violations in neighboring countries.

Since the beginning of 2018 there have been 101 cases of Vietnamese boats intruding into the waters of other countries.

A total of 163 vessels and 1,258 fishermen were detained, Nguyen Quang Hung, deputy head of the Directorate of Fisheries, said last month.

Despite its 3,260 km (2,025 miles) coastline, Vietnam is running out of near-shore seafood resources, and some fishermen who have sailed into other countries’ waters also said they face threats from Chinese vessels that are illegally anchored in Vietnamese waters for trawling and are chased away by them.