BEIJING (Reuters) – China will begin preparatory work this year for an environmental monitoring station on Scarborough Shoal in the South China Sea, an official said, as two U.S. senators introduced a bill to impose sanctions on its activities in the disputed waterway.

Last month, a Philippine minister said Chinese President Xi Jinping had promised his Philippine counterpart China would not build structures on the rocky outcrop both countries claim, but China called the comments “baffling and regrettable”.

China seized the shoal, which is northeast of the Spratly islands, in 2012 and denied access to Philippine fishermen. But after President Rodrigo Duterte visited China last year, it allowed them to return to the traditional fishing area.

This week, Xiao Jie, the mayor of what China calls Sansha City, said China planned to begin preparatory work this year to build environmental monitoring stations on a number of islands, including Scarborough Shoal.

Sansha City is the name China has given to an administrative base for the South China Sea islands and reefs it controls.

The monitoring stations, along with docks and other infrastructure, form part of island restoration and erosion prevention efforts planned for 2017, Xiao told the official Hainan Daily in an interview.

A spokesman for the Philippine foreign ministry, Charles Jose, declined to comment, saying it was trying to verify the reports.

U.S. Secretary of State Rex Tillerson arrives in Beijing on Saturday for a two-day visit, where the South China Sea, almost all of which is claimed by China, is likely to figure.

Brunei, Malaysia, the Philippines, Taiwan and Vietnam also claim parts of the waters, which have rich fishing grounds, along with oil and gas deposits. About $5 trillion worth of trade passes through the waterway each year.

The United States has criticized China’s construction of manmade islands and its build-up of military facilities there, expressing concern they could be used to restrict free movement.

This week, U.S. Senators Marco Rubio and Ben Cardin introduced the South China Sea and East China Sea Sanctions Act that would ban visas for Chinese people contributing to building development projects in the South and East China Seas.

It would also put sanctions on foreign financial bodies that “knowingly conduct or facilitate a significant financial transaction for sanctioned individuals and entities” if China steps up activity at Scarborough Shoal, among other actions.

The senators’ proposal was “extremely grating,” Chinese Foreign Ministry spokeswoman Hua Chunying said on Friday.

“I think the proposal put forward by individual senators shows their arrogance and ignorance,” Hua told a daily news briefing in Beijing.

China resolutely opposes the proposal, which infringes international law and international relations norms, she added.

Tension over the South China Sea reached a flashpoint after the Philippines filed an arbitration case against China in the Hague and as China started militarizing artificial islands it built up on reefs in the region.

China is also involved in a separate dispute with Japan over a group of uninhabited islands in the East China Sea.

(Reporting by Ben Blanchard and Christian Shepherd; Additional reporting by Manuel Mogato in MANILA; Editing by Clarence Fernandez)