"The United States 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", Ortagus said in a statement.
The spokeswoman did not name any particular incident but said China had interfered with oil and gas exploration and production of Vietnam. She quoted Secretary of State Mike Pompeo who had accused China of preventing Southeast Asian nations of accessing over $2.5 trillion in energy resources.
"China's reclamation and militarisation of disputed outposts in the SCS, 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", she added.
Ortagus also slammed China for pressuring the countries of the Association of Southeast Asian Nations to accept Code of Conduct provisions that would restrict their right to partner up with third-party companies or countries to extract resources.
The South China Sea is a major transit route which has long been subject to overlapping sovereignty claims by China, the Philippines and Taiwan. It is also believed to contain large amounts of as-yet unexplored oil and natural gas deposits.
The United States and its allies, such as the UK, France, Canada, Australia and Japan, have repeatedly challenged China’s territorial and maritime claims by conducting so-called freedom of navigation operations, sailing military vessels in the South China Sea.