Vietnamese and Chinese ships have been embroiled in a weeks-long standoff near an offshore oil block in disputed waters of the South China Sea, which fall within Vietnam's exclusive economic zone, two Washington-based think-tanks said on Wednesday.

China's U-shaped "nine-dash line" marks a vast expanse of the South China Sea that it claims, including large swathes of Vietnam's continental shelf where it has awarded oil concessions.

The Haiyang Dizhi 8, a ship operated by the China Geological Survey, on Monday completed a 12-day survey of waters near the disputed Spratly Islands, according to separate reports by the Centre for Strategic and International Studies (CSIS) and the Centre for Advanced Defence Studies (C4ADS)

One of the oil blocks it surveyed is licensed by Vietnam to Spanish energy firm Repsol, which was forced last year and in 2017 to cease operations in Vietnamese waters because of pressure from China.

As the Haiyang Dizhi 8 conducted its survey, nine Vietnamese vessels closely followed it. The Chinese ship was escorted by three China coast guard vessels, according to data from Winward Maritime, compiled by C4ADS.

In a separate incident days earlier, the China coast guard ship Haijing 35111 manoeuvred in what CSIS described as a "threatening manner" towards Vietnamese vessels servicing a Japanese-owned oil rig, the Hakuryu-5, leased by Russian state oil firm Rosneft in Vietnam's Block 06.1, 370 km southeast of Vietnam.

That block is within the area outlined by China's "nine-dash line". A series of dashes on Chinese maps, the line is not continuous, making China's claims often ambiguous.

Last year, Reuters reported that Rosneft Vietnam BV, a unit of Rosneft, was concerned that its drilling in Block 06.1 would upset China.

"On July 2 the vessels were leaving the Hakuryu-5 when the 35111 manoeuvred between them at high speed, passing within 100 metres of each ship and less than half a nautical mile from the rig," CSIS said in its report.

It was not clear on Wednesday if any Chinese ships were still challenging the Rosneft rig.

In 2014, tension between Vietnam and China rose to its highest levels in decades when a Chinese oil rig started drilling in Vietnamese waters. The incident triggered boat rammings by both sides, and anti-China riots in Vietnam.

'Ready to fight'

In response to reports of this month's standoff, which first emerged on social media, Chinese foreign ministry spokesman Geng Shuang said on July 12 that China's position on the South China Sea was "clear and consistent".

"China resolutely safeguards its sovereignty in the South China Sea and maritime rights, and at the same time upholds controlling disputes with relevant countries via negotiations and consultations," Geng said.

On Tuesday, Vietnam's foreign ministry released a statement in response to unspecified "recent developments" in the South China Sea.

"Without Vietnam's permission, all actions undertaken by foreign parties in Vietnamese waters have no legal effect, and constitute encroachments in Vietnamese waters, and violations of international law," foreign ministry spokeswoman Le Thi Thu Hang said.

Neither statements confirmed or elaborated on the standoff.

Neither Rosneft nor Repsol immediately responded to an emailed request from Reuters for comment.

In a statement on Wednesday, China's foreign ministry spokesman Geng acknowledged that there had been an incident with Vietnam.

"We hope the Vietnam side can earnestly respect China's sovereignty, rights, and jurisdiction over the relevant waters, and not take any actions that could complicate the situation," Geng told a regular news conference.

On July 11, as China was conducting its survey of the blocks, Vietnam's prime minister, Nguyen Xuan Phuc (above), visited the headquarters of the Vietnam coast guard in Hanoi.

State media did not mention the incident, but showed Phuc speaking to sailors on board vessels via a video link.

Phuc told the sailors to "stay vigilant and ready to fight" and to be aware of "unpredictable developments", the Vietnam coast guard said in a statement on its website.

On the same day, Vietnam's national assembly chairwoman, Nguyen Thi Kim Ngan, met her Chinese counterpart, Li Zhanshu, in Beijing, China's Xinhua news agency reported.

The two officials agreed to "jointly safeguard peace and stability at sea", Xinhua said.

- Reuters