Australia on Monday announced it would buy up to five US nuclear-powered submarines, then build a new model with US and British technology under an ambitious plan to bulk up Western muscle across the Asia-Pacific in the face of a rising China.

US President Joe Biden has stressed that Australia, which joined the alliance with Washington and London known as AUKUS 18 months ago, will not be getting nuclear weapons.

"Ultimately, the agency must ensure that no proliferation risks will emanate from this project," International Atomic Energy Agency chief Rafael Grossi said.

"The legal obligations of the parties and the non-proliferation aspects are paramount," Grossi added.

The United Kingdom and the United States, both nuclear-weapons states, have to report to the IAEA "international transfers of nuclear material" to non-nuclear-weapon states such as Australia, the press release said.

Australia, for its part, will have to make "an arrangement" with the UN watchdog to be able to use nuclear material "such as nuclear propulsion for submarines", Grossi stressed.

"This process involves serious legal and complex technical matters," Grossi said while promising to fulfill his mandate of verification and non-proliferation "in an impartial manner" and in all transparency.

As the discussions with the AUKUS parties continue, Grossi plans to submit a report on the subject at the next IAEA Board of Governors in June.