WASHINGTON (Reuters) - The U.S. State Department said on Saturday it is concerned by reports of China’s interference with oil and gas activities in the South China Sea, including Vietnam’s long-standing exploration and production activities.

“China’s repeated provocative actions aimed at the offshore oil and gas development of other claimant states threatens regional energy security and undermines the free and open Indo-Pacific energy market,” the State Department said in a statement.

Vietnam on Friday accused a Chinese oil survey vessel and its escorts of violating its sovereignty and demanded that China remove the ships from Vietnamese waters.

Vietnam and China have for years long been embroiled in a dispute over the potentially energy-rich stretch of waters in the South China Sea.

On Wednesday, two U.S.-based think tanks reported that Chinese and Vietnamese vessels had engaged in a standoff lasting several weeks near an oil block in Vietnam’s exclusive economic zone. Neither Beijing nor Hanoi have directly confirmed or denied the reports.

“The United States firmly opposes coercion and intimidation by any claimant to assert its territorial or maritime claims,” the State Department said.

“China should cease its bullying behavior and refrain from engaging in this type of provocative and destabilizing activity,” it added.

The State Department also pointed to Secretary of State Mike Pompeo’s comments earlier this year when he said, “by blocking development in the South China Sea through coercive means, China prevents ASEAN members from accessing more than $2.5 trillion in recoverable energy reserves.”

The State Department also said that China’s growing pressure on ASEAN countries to accept provisions that seek to restrict their right to partner with third party companies or countries further reveal its intent to assert control over oil and gas resources in the South China Sea.

“China’s reclamation and militarization of disputed outposts in the South China Sea...including the use of maritime militia to intimidate, coerce, and threaten other nations, undermine the peace and security of the region,” it said.