Vietnam on Monday demanded that China stop sending cruise ships to the South China Sea in a response to one of Beijing’s latest steps to bolster its claims in the strategic waterway.

A Chinese cruise ship with more than 300 passengers visited the disputed Paracel Islands earlier this month.

“Vietnam strongly opposes this and demands that China respect Vietnam’s sovereignty over the Paracel Islands and international law and immediately stop and not repeat those activities,” foreign ministry spokesperson Le Hai Binh told Reuters.

“Those actions have seriously violated Vietnam’s sovereignty over the Paracel Islands and international law.”

China claims 90 percent of the potentially energy-rich South China Sea. Brunei, Malaysia, the Philippines, Vietnam and Taiwan lay claim to parts of the sea, through which passes about $5 trillion of trade a year.

On Monday, Chinese foreign ministry spokeswomen Hua Chunying told a regular news briefing that she had not seen the comments by Vietnam, but reiterated China’s claim to the Paracel Islands.

Countries competing to cement their rival claims have encouraged a growing civilian presence on disputed islands in the South China Sea. The first cruises from China to the Paracel islands were launched by Hainan Strait Shipping Co in 2013.

Since 2013, two leisure cruise liners based out of what China calls Sansha city have made 120 trips around the Paracels, city mayor Xiao Jie said last Tuesday, on the sidelines of China’s annual meeting of parliament in Beijing.

In 2012, China set up Sansha, based on Woody Island in the Paracels, to administer its islands in the South China Sea, but its permanent population is no more than a few thousand and many surrounding disputed islets and reefs are uninhabited.

“The last three years have shown that Paracel Island cruises are a very suitable type of tourism for Sansha, so Sansha in the future will press on with this type (of tourism), and will proactively expand trip routes,” Xiao said.

The Paracels are also claimed by Vietnam and Taiwan.

(Reporting by My Pham; Additional reporting by Christian Shepherd in BEIJING; Editing by Michael Perry and Clarence Fernandez)