The U.S. government said Thursday that it is taking further actions to preserve a "free and open" South China Sea such as by adding major oil company China National Offshore Oil Corp. to its economic blacklist.

The measure, which came just days before the end of the administration of President Donald Trump, was taken against the company in light of its role in Beijing's campaign of "coercion against other claimants of an estimated $2.5 trillion in South China Sea oil and gas resources," Secretary of State Mike Pompeo said.

"The United States stands with Southeast Asian claimant states seeking to defend their sovereign rights and interests," Pompeo said in a statement, while also announcing visa restrictions on Chinese executives of state-owned enterprises as well as officials of the Chinese Communist Party and its military responsible for the militarization of the disputed islands in the waters.

Photo taken July 15, 2014, shows a Vietnamese patrol ship, with a Chinese oil drilling rig visible in the background, in the South China Sea near the Paracel Islands claimed by China and Vietnam. (Kyodo)

In July, the Trump administration toughened its policy on the South China Sea, calling Beijing's claims to offshore resources across most of the waters "completely unlawful."

Beijing has overlapping territorial claims with Brunei, Malaysia, the Philippines, Vietnam and Taiwan in the South China Sea.

Commerce Secretary Wilbur Ross said in a video posted by Bloomberg that dealing with the South China Sea is "a little bit like a whack-a-mole" and that additional measures are needed to tackle new evasions.

He emphasized that the latest blacklisting measure will not adversely affect legitimate Western oil companies and others that sell product to the Chinese company.

"This is specifically designed to discourage CNOOC from abusing the maritime situation," Ross added, noting that the company's enormous drilling rigs are seen accompanied by a flotilla of vessels from the Chinese coast guard and the People's Liberation Army Navy.

The company is working as a "very active and militarily supported arm of the Chinese government in order to help China take better control of natural resources that really belong to other countries," he said.

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