The State Department has warned China to halt "bullying" in the South China Sea as part of a dispute with Vietnam over oil and gas exploration in the disputed waterway.
In a blunt statement issued Saturday, department spokeswoman Morgan Ortagus said China was interfering with Vietnam's "long-standing" exploration and production of undersea energy resources.
"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," Ortagus said.
"China should cease its bullying behavior and refrain from engaging in this type of provocative and destabilizing activity," she stated.
Secretary of State Mike Pompeo said earlier this year that China has been blocking development in the South China Sea through coercive measures and as a result is preventing regional states from developing an estimated $2.5 trillion in energy reserves.
"China's reclamation and militarization of disputed outposts in the [South China Sea], along with other efforts to assert its unlawful SCS maritime claims, including the use of maritime militia to intimidate, coerce, and threaten other nations, undermine the peace and security of the region," Ortagus said.
Additionally, China has been pressuring members of the Association of Southeast Asian Nations (ASEAN) to accept an agreement that would restrict regional states' abilities to partner with third party companies and countries.
The coercion is an indication Beijing is seeking "to assert control over oil and gas resources in the South China Sea," the spokeswoman said.
"The United States firmly opposes coercion and intimidation by any claimant to assert its territorial or maritime claims," she said.
The sea has become a flashpoint in U.S.-China relations, along with trade disputes over Chinese technology theft and unfair trade practices.
China tried to claim 90 percent of the sea as historical maritime territory in an unprecedented bid to take control of the international waterway.
Three years ago the U.N. Permanent Court of Arbitration ruled the Chinese claim to own the South China Sea, based on a vaguely defined Nine-Dash Line around the waterway, was illegal. China has rejected the court ruling.
Adm. Philip Davidson, commander of the Indo-Pacific Command, said last week that China conducted a series of provocative missile tests in the South China Sea as part of messages to the United States by China's defense minister.
The Chinese fired six anti-ship ballistic missiles in the South China Sea in the first sea launch of the new DF-21D missile. The unique missile is designed to sink U.S. aircraft carriers at sea with a maneuvering warhead.
The Pentagon also said the missile tests were a violation of a 2015 pledge made by Chinese president Xi Jinping not to militarize disputed islands in the South China Sea.
The U.S. statement came amid growing tensions between Vietnam and China over access to undersea resources.
Vietnam's government on July 20 called on China to remove a survey vessel and coast guard ships near Vanguard Bank, a reef in the western part of the Spratly Islands that is claimed by China, Vietnam, Philippines, and other states.
In early July, China's survey ship and accompanying coast guard ships began conducting undersea seismic tests.
Vietnam responded by sending its coast guard vessels to the region.
Vietnamese Foreign Ministry spokeswoman Le Thi Thu Hang said July 19 that the Chinese survey ship Haiyang Dizhi 8 and escort vessels had carried out activities in violation of Vietnam's exclusive economic zone and continental shelf.
"Vietnam has made contact with China on multiple occasions via different channels, delivered diplomatic notes to oppose China's violations, and staunchly demanded China to stop all unlawful activities and withdraw its ships from Vietnamese waters," the ministry statement said.
The dispute over undersea exploration is the most serious since May 2014, when China dispatched a mobile oil platform to the Paracel Islands. During that standoff, Vietnam sent vessels that sought to prevent the Chinese platform from attaching to the seabed and were challenged by Chinese escort ships.
In Beijing, Foreign Ministry spokesman Geng Shuang accused the United States of meddling in the South China Sea energy dispute.
"For a long time, external forces including the U.S. have been making wanton remarks on this issue, stirring up troubles and sowing discord with ill intentions," Geng said Monday.
Recent comments by Pompeo and White House national security adviser John Bolton on the South China Sea were "slanders" against China.
"We urge the U.S. to stop such irresponsible behaviors, respect the facts and the efforts of China and ASEAN countries to settle disputes through dialogue, and make positive contributions to peace and stability in the South China Sea rather than doing the opposite," he said.
Further north, Chinese and Russian bombers and aircraft were confronted by jet interceptors from South Korea and Japan, after the Chinese and Russian aircraft intruded in Seoul's air defense zone over islands in the Sea of Japan.
The bombers were described as Chinese H-6s and Russian Tu-95s.
South Korea's military also said more than 300 warning shots were fired at a Russian A-50 command and control military jet that intruded into South Korean airspace.
The four-nation standoff was the first of its kind in the region.