HANOI • A Chinese survey vessel has re-entered disputed waters in the South China Sea, according to ship tracking analysis, after a tense month-long stand-off in the same area that inflamed tensions between Hanoi and Beijing.

China has been accused of deploying warships, arming outposts and ramming fishing vessels in the waters, stoking ire from other claimants on the key global shipping route.

The resource-rich sea is also a flashpoint issue between Beijing and Washington; the US has slammed China for its behaviour and called for maritime freedom.

Last month, Chinese geological survey ship Haiyang Dizhi 8 entered waters surrounding the Spratly Islands, which are also claimed by Vietnam. It remained for several weeks with a number of coast guard ships. Hanoi said last week that the ship had left the area, but it returned on Tuesday, according to the United States-based Centre for Advanced Defence Studies (C4ADS).

The ship was accompanied by at least two Chinese coast guard vessels, and several Vietnamese ships followed the Chinese fleet.

It remained in the area early yesterday, C4ADS senior analyst Devin Thorne said, adding that the survey ship's operations in the area "reflect China's purposeful use of civilian, commercial, scientific and paramilitary resources to pursue its interests in and vision for the South China Sea".

The US lashed out at China for its "bullying behaviour" in the South China Sea, after reports emerged last month of the survey ship in what Vietnam says is its exclusive economic zone.

US Secretary of State Mike Pompeo last month accused Beijing of "coercion" in the sea and urged South-east Asian nations to stand up to the superpower.

Beijing often invokes its so-called nine-dash line to justify its apparent historic rights over most of the South China Sea, parts of which are also claimed by Taiwan, Malaysia, the Philippines and Brunei.

Hanoi and Beijing have long sparred over the area, with tensions coming to a head in 2014, when China moved an oil rig into waters claimed by Vietnam. That sparked weeks of deadly nationwide protests across Vietnam, unleashing deep-seated anti-China anger.

AGENCE FRANCE-PRESSE