The United States has weighed in on an ongoing stand-off between Vietnam and China over Chinese operations in the South China Sea, urging Beijing to cease interfering with oil and gas activities in the disputed waters.
The statement from the US State Department on Saturday came a day after Hanoi accused a Chinese oil exploration ship and other vessels of operating in waters in Vietnam's exclusive economic zone, and demanded their removal.
China has not responded so far.
The stand-off involved nine Vietnamese vessels and three Chinese coast guard vessels that were escorting the oil exploration ship, said two Washington-based think-tanks last Wednesday.
State Department spokesman Morgan Ortagus said in Saturday's statement: "China's repeated provocative actions aimed at the offshore oil and gas development of other claimant states threaten regional energy security and undermine the free and open Indo-Pacific energy market."
The US has repeatedly spoken out against what it sees as China's increasingly muscular assertion of its claims to the South China Sea.
Beijing claims nearly all the waters, including territorial waters of Malaysia, the Philippines and Vietnam, despite a 2016 ruling from an independent arbitration tribunal that its claims were legally baseless.
US Secretary of State Mike Pompeo said in March that China, by blocking development in the South China Sea through coercive measures, was preventing Asean members from accessing more than US$2.5 trillion (S$3.4 trillion) in recoverable energy reserves.
THREAT TO ENERGY SECURITY
China's repeated provocative actions aimed at the offshore oil and gas development of other claimant states threaten regional energy security and undermine the free and open Indo-Pacific energy market.
STATE DEPARTMENT SPOKESMAN MORGAN ORTAGUS
On Saturday, Ms Ortagus said China's reclamation and militarisation of disputed outposts in the South China Sea, including its use of maritime militia to intimidate, coerce and threaten other countries, "undermine the peace and security of the region".
"The US firmly opposes coercion and intimidation by any claimant to assert its territorial or maritime claims. China should cease its bullying behaviour and refrain from engaging in this type of provocative and destabilising activity."
She echoed US National Security Adviser John Bolton, who wrote on Twitter last Friday: "China's coercive behaviour towards its South-east Asian neighbours is counterproductive and threatens regional peace and stability."
Mr Gregory Poling, director of the Washington-based Asia Maritime Transparency Initiative, called the statement an "overdue change of tone" from Washington's singular focus on the freedom of navigation, and made clear that the US cared about all lawful uses of the sea.
"The State Department takes an important step with today's statement on Chinese harassment of Vietnam oil and gas ops and use of militia to destabilise the South China Sea," he wrote on Twitter.
The statement comes as the Asean grouping grapples with charting a middle course between the US and China amid rising tensions between the two superpowers.
Asean's 10 members will meet dialogue partners, including the US and China, next week in Bangkok at the annual Asean Regional Forum to discuss regional issues. Asean and China are in the midst of negotiations on a Code of Conduct to manage tensions in the South China Sea.
However, Ms Ortagus said: "China's growing pressure on Asean countries to accept Code of Conduct provisions that seek to restrict their right to partner with third-party companies or countries further reveals its intent to assert control over oil and gas resources in the South China Sea."