(Bloomberg) -- The U.S. Commerce Department announced Thursday that it’s blacklisting China’s third-biggest oil company after years of involvement in offshore drilling in disputed South China Sea waters.

The move denies China National Offshore Oil Corp., the nation’s main deepwater explorer, from access to U.S. technologies. It comes as part of a Trump administration push against Beijing during its final days in office, following a decision to blacklist more than 60 other Chinese companies in December. It was not immediately clear why Cnooc had been added now after it was not included in earlier listings.

“China’s reckless and belligerent actions in the South China Sea and its aggressive push to acquire sensitive intellectual property and technology for its militarization efforts are a threat to U.S. national security,” Commerce Secretary Wilbur Ross said in a statement. “CNOOC acts as a bully for the People’s Liberation Army to intimidate China’s neighbors, and the Chinese military continues to benefit from government civil-military fusion policies for malign purposes.”

Cnooc is the smallest of China’s so-called big three state-owned oil majors after China National Petroleum Corp. and China Petrochemical Corp., also known as Sinopec. The company’s operations in the South China Sea have proved controversial with neighbors because China claims drilling rights in waters far from its borders, and within 200 miles of countries like Vietnam and the Philippines.

Cnooc has been at the center of territorial disputes in the region since 2012, when it invited foreign drillers to explore blocks off Vietnam that Hanoi’s leaders had already awarded to companies including Exxon Mobil Corp. and OAO Gazprom.

The Commerce Department also added Skyrizon to the military end-user list, saying it poses a threat to national security. Ross said the company’s push to acquire and “indigenize” foreign military technologies posed a significant threat to U.S. national security and foreign policy interests.

“This action serves to warn the export community of Skyrizon’s significant ties to the People’s Liberation Army,” Ross said.

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