The Da Yang Hao docked in the palm-shaded port of Avatiu on a "scientific research cruise" through the tropical archipelago, the Cook Islands' Seabed Minerals Authority said.

Huge areas of seabed around the Cook Islands are carpeted in polymetallic nodules, lumpy rocks studded with rare earths and critical minerals such as cobalt, nickel and manganese.

The Cook Islands -- which lays claim to one of the world's biggest deposits of polymetallic nodules -- signed a contentious deep-sea mining cooperation deal with China earlier this year.

"This cruise is about learning by doing," said Seabed Minerals Authority spokesman Edward Herman.

"Our team is actively participating in all activities to build our knowledge and capacity in marine research."

This included mapping the seabed with sonar arrays and dredging up sediment samples, said the authority.

Fringed by sparkling lagoons, South Pacific nation the Cook Islands has opened its vast ocean territory for deep-sea mining exploration.

With existing supplies of critical minerals heavily exposed to trade disputes, both the United States and China have this year ramped up interest in the highly contentious industry.

Critical minerals, such as cobalt and nickel, are in hot demand for electric vehicles, rechargeable batteries and advanced military technology.

The United States secured its own agreement to pursue deep-sea mining research with the Cook Islands in August.

- Deep-sea exploration -

AFP photos showed the near-100 metre (330 feet) Da Yang Hao vessel cutting through the Cook Islands' sleepy main harbour before docking on Saturday morning.

The ship was emblazoned with the logo of China's Ocean Mineral Resources Research Association, which is tasked with the exploration of seabed minerals.

Chinese research vessels have been accused of carrying out surveillance and other military activities under the auspices of scientific work.

Pacific island nation Palau, a staunch US ally in the region, in 2021 alleged that the Da Yang Hao had entered its exclusive economic zone without proper permissions.

The Da Yang Hao arrives in the Cook Islands less than one month after a deep-sea research voyage backed by the US National Oceanic and Atmospheric Administration.

While exploration for deep-sea mining is far advanced, no company or nation has started production on a commercial scale.

Critics fear deep-sea mining will smother marine life with waste and the noise of heavy machinery will disrupt oceanic migrations.

International regulators are still trying to thrash out environmental rules governing the industry.