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, reports Trend citing to Reuters.

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 Center for Strategic and International Studies (CSIS) and the Center for Advanced Defense 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 maneuvered in a "threatening manner” toward 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 (230 miles) 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 exclusively 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 maneuvered between them at high speed, passing within 100 meters 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.