Presence last month of Haiyang Dizhi 8 survey ship in waters around Spratly Islands also drew protest from 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 standoff in the same area that inflamed tensions between Vietnam and China.
Vietnam said last week the ship had left the area, but on Tuesday it had returned, according to the US-based Center for Advanced Defense Studies (C4ADS), and has remained in the area early on Wednesday.
The ship was accompanied by at least two Chinese coastguard ships, and several Vietnamese ships closely followed the Chinese fleet.
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, which 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 claimed by Vietnam.
It remained for several weeks with a number of coastguard ships.
C4ADS senior analyst Devin Thorne said 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”.
Vietnamese officials did not immediately respond to AFP news agency’s request for comment.
The United States lashed out at China for its “bullying behaviour” in the South China Sea after initial reports emerged of the survey ship in what Vietnam says is its exclusive economic zone last month.
US Secretary of State Mike Pompeo in July accused Beijing of “coercion” in the sea and urged Southeast Asian nations to stand up to the superpower.
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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.
The nine-dash line claim has long been declared invalid by the International Court of Arbitration in The Hague.
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 incident sparked weeks of deadly nationwide protests across Vietnam, unleashing deep-seated anti-China anger.
Separately, Beijing is also being accused of intruding into waters within the exclusive economic zone of the Philippines.
In recent weeks, it has been reported that Chinese survey ships are entering Philippine territory without permission from Manila. The incident prompted the Philippines to ban all foreign marine survey ships from its territory on Monday.