Last year, Vietnamese vessels spent months shadowing the Chinese Haiyang Dizhi 8 survey vessel in resource-rich waters which are a future global flashpoint as the US challenges the reaching maritime claims of China.

On Tuesday, the ship, which is used for seismic offshore surveys, popped up again 158 km (98 miles) off the coast of Vietnam, within its exclusive economic zone (EEZ), surrounded by at least one China Coast Guard vessel, according to Marine Traffic data, a website that tracks shipment.

It also follows this month's sinking of a Vietnamese fishing boat close to islands in the disputed waters, an act that drew demonstrations from Vietnam and accusations that China had violated its sovereign rights and threatened its fishermen's lives.

"We call on the PRC to remain focused on supporting international efforts to combat the global pandemic, and to stop exploiting the distraction or vulnerability of other states to expand its unlawful claims in the South China Sea," the U.S. State Department stated.

The Philippines, which has also contested claims in the South China Sea, has also raised questions.

China's Global Times, released by China's ruling Communist Party's official People's Daily newspaper, said Vietnam had used the fishing boat incident to distract from its "ineptitude" when dealing with coronavirus.

For years, China and Vietnam have been at loggerheads over possibly energy-rich waters, which Vietnam calls the East Sea.

China's U-shaped "nine-dash line" on its map data marks a huge expanse of the waters it claims, including large portions of Vietnam's continental shelf where oil concessions have been awarded. Malaysia and Brunei claim some of the waters southwards claimed by China.

Maritime Business World