China "has decided to present its candidature of the city of Xiamen to host the Secretariat" of the treaty, the Chinese mission to the United Nations wrote in a letter to Secretary-General Antonio Guterres, according to a copy seen by AFP.

The treaty will officially enter into force on Saturday, and the host country of the eventual secretariat will be decided later this year.

Until now, Belgium and Chile had been vying to host the future organization.

The Xiamen bid signals "China's intention to help shape global rules," said Li Shuo, director of the China Climate Hub at the Asia Society Policy Institute in Washington, calling it a "notable move."

China's announcement came just days after US President Donald Trump announced his country will withdraw from 66 global organizations and treaties -- involving UN and non-UN entities.

They include the United Nations Framework Convention on Climate Change (UNFCCC), the parent treaty underpinning all major international climate agreements, ratified by almost every country in the world.

After years of delays, the treaty to protect the high seas was ratified in September by 60 countries. The law aims to protect biodiverse areas in waters worldwide, extending beyond countries' exclusive economic zones.

Teeming with plant and animal life, the oceans are responsible for creating half of the globe's oxygen supply and are vital to combatting climate change, conservationists say.

Once the treaty becomes law, a decision-making body will have to work with a patchwork of regional and global organizations already overseeing different aspects of the oceans.

These include regional fisheries bodies and the International Seabed Authority -- the forum where nations are jousting over proposed rules on the environmentally destructive deep-sea mining industry.