State Department spokeswoman Morgan Ortagus said in a statement that 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."
Vietnam on Friday demanded China remove a survey ship from Vanguard Bank, which it says lies within Vietnam's 200-mile exclusive economic zone. China claims the South China Sea almost in its entirety and has rattled smaller neighbors by constructing seven man-made islands in the disputed waters and equipped them with military runways and outposts.
Chinese coast guard vessels also have been reported near a drilling rig in the same Vanguard Bank area where Vietnam has contracted Russia's Rosneft to develop gas fields.
"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," Vietnam's Foreign Ministry spokeswoman Le Thi Thu Hang said in a statement Friday.
Chinese Foreign Ministry spokesman Geng Shuang earlier in the week urged Hanoi to respect China's sovereign rights and jurisdiction, "and not to take any move that may complicate matters."
In May 2014, Chinese and Vietnamese vessels engaged in a dangerous confrontation when China's national oil company moved its oil platform into waters Vietnam considers its territory.
Ortagus calls on China to "cease its bullying behavior and refrain from engaging in this type of provocative and destabilizing activities."