Ortagus said the US was "seriously concerned" on learning about the incident that took place near the Paracel Islands in the South China Sea. He said the the incident was another act by China to assert "unlawful maritime claims and disadvantage its Southeast Asian neighbors in the South China Sea."

On Thursday, a Chinese coast guard vessel rammed and sank the boat owned by Tran Hong Tho, a native of the central province of Quang Ngai, with eight fishermen onboard as they were fishing off the Phu Lam (Woody) Island in Vietnam’s Hoang Sa Archipelago (Paracel Islands). Vietnam calls the South China Sea the East Sea.

The U.S. State Department spokesman also noted that Beijing has rolled out new research stations on military bases it has built on Fiery Cross Reef and Subi Reef, and landed special military aircraft on the Fiery Cross Reef since the onset of the global pandemic.

Maritime militia has also continued to be deployed by China around the Spratly Islands, Ortagus said, adding that China’s infamous nine-dashed line had been deemed "an unlawful maritime claim by an arbitral tribunal convened under the 1982 Law of the Sea Convention in July 2016, a position shared by the U.S. Government."

Ortagus called on Beijing to remain focused on working on the Covid-19 pandemic with the international community and to cease its exploitation of "the distraction or vulnerability" of other countries to expand its claims in the South China Sea.

Vietnamese Foreign Ministry spokesman Le Thi Thu Hang had demanded Friday that China investigates the incident and " take strict action against the Chinese vessel and refrain from repeating such acts in future as well as pay proper compensation for the damage caused to the Vietnamese fishermen."

"These actions of the Chinese vessel have infringed on Vietnam's sovereignty over the Hoang Sa Islands and endangered the lives and properties of Vietnamese fishermen," she said.

China's actions also go against the common view of senior leaders of the two countries on treating fishermen humanely; as well as the Vietnam-China Agreement on Basic Principles Guiding the Settlement of Maritime Issues. They also violate the Declaration on the Conduct of Parties in the South China Sea and negatively affect negotiations for a Code of Conduct in the waters, Hang said.

The move has increased tensions, caused destabilization and is not beneficial to the maintenance of peace, stability and cooperation in the region, she added.

Hang had reiterated that Vietnam has full legal basis and historical evidence under international law to assert its sovereignty over the Hoang Sa (Paracel) and Truong Sa (Spratly) archipelagos.

The Vietnam Fisheries Society has also called on the Government Office, the Ministry of Foreign Affairs and other government agencies to ensure China pays compensation to Vietnamese fishermen for sinking their boats.

The Quang Ngai’s Fisheries Society said that after sinking the Vietnamese boat, the Chinese vessel had picked up eight Vietnamese fishermen and taken them to the Woody Island. Upon receiving news of the sunken ship, three other Vietnamese fishing boats tried to rescue then, but were chased away by the Chinese vessel. Two of the vessels were captured and towed to the island.

At 2 p.m. Thursday, China released the two fishing boats and eight fishermen. A day later, Vietnamese authorities received the detained Vietnamese fishermen safely.

China seized the Paracel Islands from South Vietnam by force in 1974, and has since been occupying them illegally.

In 2014 China brought an oil rig, Haiyang Shiyou 981, and installed it in waters off the Paracel Islands, changing the status quo in the waters. Since then, Chinese ships have chased after and rammed many Vietnamese fishing vessels near the islands in Vietnamese waters.