China has clashed with the countries in the AUKUS alliance at a meeting of the UN nuclear watchdog over their plan to supply Australia with nuclear-powered submarines, capping a week in which Beijing has repeatedly railed against the project.
Under the alliance between Washington, London and Canberra announced last year, Australia plans to acquire at least eight nuclear submarines that International Atomic Energy Agency (IAEA) chief Rafael Grossi has said will be fuelled by "very highly enriched uranium", suggesting it could be weapons-grade or close to it.
To date no party to the nuclear Non-Proliferation Treaty (NPT) other than the five countries the treaty recognises as weapons states - the United States, Russia, China, Britain and France - has nuclear submarines.
The vessels can stay underwater for longer than conventional subs and are harder to detect.
"The AUKUS partnership involves the illegal transfer of nuclear weapon materials, making it essentially an act of nuclear proliferation," China said in a position paper sent to IAEA member states during this week's quarterly meeting of the IAEA's 35-nation Board of Governors.
Australia says it will be unable and unwilling to use the fuel in its submarines to make nuclear weapons.
The vessels will have "welded power units" containing nuclear material that would need chemical processing for use in an atom bomb, and Australia does not have or want facilities that can do that.
The AUKUS countries and the IAEA say the NPT allows so-called marine nuclear propulsion provided necessary arrangements are made with the IAEA.
China disagrees in this case because nuclear material will be transferred to Australia rather than being produced by it.
It argues the IAEA is overstepping its mandate and wants an unspecified "inter-governmental" process to examine the issue at the IAEA instead of leaving it to the agency.
In its seven-page position paper, China said AUKUS countries were seeking to take the IAEA "hostage" so it could "whitewash" nuclear proliferation.
Nuclear submarines are a particular challenge because when they are at sea their fuel is beyond the reach of the agency's inspectors who are supposed to keep track of all nuclear material.
IAEA chief Grossi has said he is satisfied with the AUKUS countries' transparency so far.
This week's clash has done little to change the way the IAEA approaches the submarine plan, which is still being developed.
But it shows China continues to campaign vocally against it, even at the risk of harming its relations with the IAEA.
"It is deeply concerning to hear China calling into question the legitimacy and integrity of the IAEA," an AUKUS statement to the Board of Governors said on Friday.
"The AUKUS partners have full confidence in the ability of the IAEA Director General and Secretariat to carry out the Agency's mission and mandate."