U.S. Secretary of State Mike Pompeo on Tuesday posted on Twitter the diplomatic note, which says: "Today, the U.S. protests the PRC’s (People’s Republic of China) unlawful South China Sea maritime claims at the U.N."

"We reject these claims as unlawful and dangerous. Member States must unite to uphold international law and freedom of the seas."

The note was sent on Monday by U.S. Ambassador Kelly Craft in response to one sent by the Permanent Mission of China to the U.N. on December 12 last year in response to Malaysia’s submission to the Commission on the Limits of the Continental Shelf.

"The U.S. rejects these maritime claims as inconsistent with international law as reflected in the 1982 Law of the Sea Convention," the note read.

China’s note had said it has "sovereignty over Nanhai Zhudao, consisting of Dongsha Qundao, Xisha Qundao, Zhongsha Qundao and Nansha Qundao," based on "historic rights in the South China Sea."

Nanhai Zhudao refers to South China Sea islands, Dongsha Qundao to Pratas Islands, Xisha Qundao to Vietnam’s Paracel Islands, Zhongsha Qundao to the Macclesfield Bank and the Scarborough Shoal, and Nansha Qundao to Vietnam’s Spratly Islands.

Vietnam rejected and protested China’s note with one of its own on March 30, which said China’s claims "seriously violate Vietnam's sovereignty and jurisdiction rights over the East Sea."

Vietnam calls the South China Sea the East Sea.

China claims almost the entire South China Sea, including waters close to Vietnam, the Philippines, Malaysia, Brunei, and Taiwan.