Officials in Vietnam accused China of having “seriously violated” the country’s sovereignty in the South China Sea, the South China Morning Post (SCMP) reported on Monday.

Vietnam’s grievance came shortly after China announced over the weekend it had established two new administrative districts to govern the Paracel Islands – which belong to Vietnam – and the Spratly Islands, the sovereign territory of the Philippines. China also declared that it had renamed 80 geographical features in the Paracel and Spratly Island chains, an act described by the Chinese Communist Party (CCP) as “reaffirming China’s sovereignty in the region.”

The two new administrative districts – which China called “Xisha” and “Nansha” – will be ruled by “Sansha,” a city manufactured by the Chinese government in 2012 purely to assert Beijing’s claims over nearly all of the South China Sea.

Vietnam’s foreign ministry spokesperson Le Thi Thu Hang issued a statement on Sunday that said China’s latest actions would only add to tense relations surrounding the highly contested waterway.

“These acts are not conducive to the development of the friendly relations between countries and further complicate the situation in the East Sea [Vietnam’s name for the South China Sea], the region and the world,” she said. “Vietnam demands that China respect Vietnam’s sovereignty and annul its wrongful decisions and not repeat similar activities in the future.”

Chinese Foreign Ministry spokesman Geng Shuang said in a press conference on Monday that China’s establishment of the new districts and the renaming of features in the South China Sea “accords with international law.”

A 2016 international court ruling declared China’s maritime claims in the South China Sea to be unlawful. China refuses to honor this ruling, pushing forward with its goal to assert power over almost the entire sea. Similarly, China rejected Vietnam’s concerns on Monday.

“China firmly opposes Vietnam’s words and actions that undermine China’s sovereignty and rights and interests in the South China Sea and will continue to take necessary measures to firmly safeguard China’s sovereignty and rights and interests,” Geng said.

In recent years, China has brazenly increased its presence in the South China Sea, a geologically rich waterway, home to oil and gas reserves and a strategic trade route. China encroaches upon international waters and illegally lays claim to maritime zones governed by Southeast Asian nations like Vietnam, the Philippines, Malaysia, Taiwan, and Brunei.

Monday’s statement by Vietnam was the second time in a month the country has issued a complaint against China’s belligerent actions in the South China Sea. On April 4, Vietnam released an official statement of protest against China after the country hit and sunk a Vietnamese fishing boat in waters off of Vietnam’s Paracel Islands on April 3.