A Chinese military helicopter flies close to a Philippine Bureau of Fisheries and Aquatic (BFAR) aircraft above Scarborough shoal on Tuesday, Feb. 18, 2025. (AP Photo/Joeal Calupitan, File)

BANGKOK (AP) — Chinese authorities said they began live-fire exercises in the Gulf of Tonkin on Monday, only days after Vietnam announced a new line marking what it considers its territory in the body of water between the countries.

China's Maritime Safety Administration said the exercises would be focused on the Beibu Gulf area, closer to the Chinese side of the Gulf of Tonkin, and would run through Thursday evening.

It gave no further details, but the drills follow an announcement last week by Vietnam establishing a baseline used to calculate the width of its territorial waters in the Gulf of Tonkin.

State-run Vietnam News reported that the baseline was in compliance with the U.N. Convention on the Law of the Sea and would provide "a robust legal basis for safeguarding and exercising Vietnam's sovereignty, sovereign rights and jurisdiction."

Vietnam has not publicly responded to the Chinese drills.

China and Vietnam have long had a maritime agreement governing the Gulf of Tonkin, but have been locked in competing claims in the nearby South China Sea over the Spratly and Paracel Islands and maritime areas.

China has been growing aggressive in pursuing those claims, and in October assaulted 10 Vietnamese fishermen near the Paracel Islands, three of whom suffered broken limbs.

China claims almost the entire South China Sea as its own, though it has not publicly released exact coordinates of its claim other than a map with 10 dashed lines broadly demarcating what it calls its territory.

In addition to Vietnam, China's claims overlap with those of the Philippines, Malaysia, Brunei and Taiwan, while Indonesia has also figured in violent confrontations with the Chinese coast guard and fishing fleets in the waters around the Natuna Islands.

Tensions have been particularly high with the Philippines, with regular confrontations between the two countries.

Most recently, a Chinese navy helicopter flew within 10 feet (3 meters) of a Philippine patrol plane last week over the South China Sea, near the hotly disputed Scarborough Shoal off the northwestern Philippines.

Leaders in Australia and New Zealand also said China should have given more warning before its navy conducted an unusual series of live-fire exercises in the seas between the two countries, forcing flights on Friday and Saturday to divert on short notice.

Political leaders from both countries emphasized that China didn't breach international law, but said they had only been given "a couple hours’ notice" rather than the usual 12 to 24 hours.