"By joining Israel's attack on Iran, the US is also breaking international law. Military action against Iran is not the way to resolve concerns over Tehran's nuclear programme," ICAN's executive director Melissa Parke said in a statement.

"Given that US intelligence agencies assess Iran is not pursuing nuclear weapons, this is a senseless and reckless act that could undermine international efforts to prevent the further proliferation of nuclear weapons."

Geneva-based ICAN won the 2017 Nobel Peace Prize for its key role in drafting the Treaty on the Prohibition of Nuclear Weapons, which took effect in 2021.

Some 69 countries have ratified it to date, four more have directly acceded to the treaty and another 25 have signed it, although none of the nuclear weapons states hae come on board.

"The US should have continued to pursue the diplomatic process under way before Israel resorted to the illegal use of force. This does not make the region or the world safer. It makes it more dangerous," Parke said.

"Striking nuclear installations is explicitly banned under international law and risks causing radioactive contamination harmful to human health and the environment. The US must stop all military action and return to the diplomatic path."

In its flagship annual report on June 13 -- the day Israel began its strikes on Iran -- ICAN said nuclear armed states spent more than $100 billion on their atomic arsenals last year.

ICAN said Britain, China, France, India, Israel, North Korea, Pakistan, Russia and the United States together spent nearly $10 billion more than in 2023.

The United States spent $56.8 billion in 2024, followed by China at $12.5 billion, Britain at $10.4 billion, Russia at $8.1 billion and France at $6.9 billion, said ICAN.