Do Van Hau, deputy chief executive of state oil and gas group Petrovietnam, said he had asked the government to make “the strongest possible” protest to China over the incident, which took place early on Thursday about 120 km (80 miles) off the south-central coast of Vietnam, state media reported.

Vietnam’s Foreign Ministry did not respond to a request for a comment. China’s Foreign Ministry had no immediate comment.

The South China Sea covers an area of more than 648,000 sq miles (1.7 million sq km), containing more than 200 mostly uninhabitable small islands, rocks and reefs.

China, Vietnam, the Philippines, Malaysia, Brunei and Taiwan all claim territories in the sea, which covers an important shipping route and is thought to hold untapped oil and gas reserves. 

The Vietnamese ship, the Binh Minh 02, detected the Chinese patrol boats approaching on radar at about 5 a.m. on Thursday(2200 GMT on Wednesday), the official Vietnam News Agency reported.

About an hour later, the three Chinese boats ran through the area where the Vietnamese ship was working, snapping cables the ship was using, then left the scene after about three hours.

The incident took place in an area called Block 148 off the coast from the beach town of Nha Trang, a map on the news agency’s website showed.

The news agency said the ship fixed the gear and resumed work on Friday. Petrovietnam said it would work closely with the government to protect the ship.