At least three Vietnamese vessels were moving with the Chinese ship, according to data issued by the Marine Traffic site.

The presence of the Haiyang Dizhi 8 in Vietnam’s EEZ comes towards the scheduled end of a 15-day nationwide lockdown in Vietnam aimed at curbing the spread of the coronavirus.

“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 US State Department said in a statement, referring to China.

On Saturday, the Global Times, published by the official People’s Daily newspaper of China’s ruling Communist Party, said Vietnam had used the fishing boat incident to distract from its “ineptitude” in handling the coronavirus.

Vietnam’s Ministry of Foreign Affairs did not immediately respond to a request for comment.

Helped by a mass quarantine and aggressive contact-tracing, Vietnam has recorded 265 cases of the novel coronavirus and no deaths. Nearly 122,000 coronavirus tests have been carried out in Vietnam.

Coronavirus: what’s behind Vietnam’s containment success?

China and Vietnam have for years been at loggerheads over the potentially energy-rich waters, called the East Sea by Vietnam.

During the stand-off last year, at least one Chinese coastguard vessel spent weeks in waters close to an oil rig in a Vietnamese oil block, operated by Russia’s Rosneft, while the Haihyang Dizhi 8 conducted suspected oil exploration surveys in large expanses of Vietnam’s EEZ.

“The deployment of the vessel is Beijing’s move to once again baselessly assert its sovereignty in the South China Sea,” said Ha Hoang Hop, at the Singapore-based ISEAS-Yusof Ishak Institute.

“China is using the coronavirus distraction to increase its assertiveness in the South China Sea, at a time when the US and Europe are struggling to cope with the new coronavirus.”