Chinese coast guard ships deployed water cannons—and a suspected sonic weapon—against a Philippine government mission to deliver crucial supplies to fishermen.

China’s maritime patrol ships deployed a suspected sonic weapon against Philippine government vessels on Saturday as they sought to obstruct a Philippines supply mission.

The assault using unspecified “acoustic devices” allegedly inflicted “severe temporary discomfort” on Philippine crew members, Manila said, and was accompanied by sustained water cannon blasts as well as maneuvering that risked a collision.

The incident happened near Scarborough Shoal, one of the most contentious features in the Philippines and China’s long-running territorial feud in the South China Sea since the latter achieved facto control there in 2012 by force.

China continues to block Philippine fishermen from accessing traditional fishing grounds at Scarborough Shoal, while the Philippines says it is their right to fish there under international law. Philippine fisheries bureau boats were attempting to deliver humanitarian supplies, including food and fuel, to the waiting fishermen.