The U.S. and Philippines have expressed concern over China’s decision to mass militia vessels in the South China Sea, as a Filipino civilian ship carrying local journalists reported it had been chased down by armed Chinese Navy vessels while sailing within its own exclusive economic zone on Thursday.

Local broadcaster ABS-CBN claims the People’s Liberation Army Navy deployed two vessels carrying missiles to drive away the ship as it traveled across reefs and shoals close to the western island province of Palawan. The report adds it was the first recorded instance of a military maneuver against a civilian boat.

After being spotted by a coast guard vessel, their ship was radioed and then pursued for an hour, "getting so close that bow number 5101 was visible to the naked eye, sometimes sailing beside the Filipino vessel on either side,” according to the report. It then turned away, only for two Houbei-class missile boats to show up moments later.

During a phone call on Friday, U.S. Secretary of State Antony Blinken and Philippine Secretary of Foreign Affairs Teodoro Locsin discussed concerns about the build-up of China’s maritime militia in the disputed waters, including Whitsun Reef, according to a U.S. readout. They reiterated calls for China to abide by the 2016 arbitration ruling issued pursuant to the Law of the Sea Convention

The growing tension comes as Chinese vessels — initially numbering in the hundreds — were spotted at Whitsun Reef, prompting protests from Manila. The U.S. last month said it stands by the Philippines while accusing China of using a "maritime militia to intimidate, provoke and threaten other nations.” China said last month that the ships were simply "taking shelter from the wind” and the Philippines should view the situation in a "rational light.”