Philippine authorities have arrested six Chinese nationals and a Filipino over suspected spying on U.S. and Philippine navy vessels at the entrance of Subic Bay, a location considered as strategically crucial.

The arrests of the Chinese suspects, who officials said were posing as fishermen, brought to 12 the number of Chinese citizens taken into custody in the Philippines this year for alleged espionage, reported Radio Free Asia.

The arrests occurred amid a strained relationship between China and the Philippines over the South China Sea.

The seven suspects were arrested on March 19 after Philippine military intelligence alerted the National Bureau of Investigation about “foreign nationals suspected of carrying out covert intelligence, surveillance and reconnaissance operations” around Grande Island at the entrance of Subic Bay, the NBI said in a statement as quoted by Radio Free Asia.

The six Chinese “were closely monitored for engaging in deemed suspicious activities, involving a collection of sensitive data” affecting national defense, the bureau said.

The Filipino national was reportedly acting as their security guard.

“Counter-intelligence efforts disclosed that these individuals were occupying the island under the guise of recreational fishers, frequently lingering at the wharves until the wee hours,” NBI director Jaime Santiago was quoted as saying by Radio Free Asia.

“However, multiple witnesses reported that the group was utilizing drones in the guise of transporting fishing bait – conducting surveillance on naval assets, including those from local forces and allied nations, passing through Grande Island,” he said.

Subic Bay, Grande Island, is considered as strategically crucial since once an artillery training ground for American forces.

From 1991 to 1992, it was the home to the largest US naval base outside  America.