US prosecutors have charged an alleged member of the Japanese mafia with conspiring to traffic nuclear materials.
Takeshi Ebisawa, 60, tried to sell uranium and plutonium that he believed would be transferred to Iran to build a nuclear bomb, it is alleged.
Mr Ebisawa and a Thai co-defendant were previously hit with weapons and drug charges in April 2022.
He faces life imprisonment if convicted of the latest charges.
US authorities say Mr Ebisawa - who is being held in a Brooklyn jail - is a senior figure in the Japanese organised crime syndicate, known as the Yakuza, with operations in Sri Lanka, Myanmar, Thailand and the US.
The US Department of Justice said Mr Ebisawa and his "confederates showed samples of nuclear materials in Thailand" to an undercover agent from the US Drug Enforcement Agency (DEA).
The agent was posing as a drugs and weapons trafficker with links to an Iranian general.
The nuclear samples - which came from Myanmar - were seized by Thai authorities and transferred to US investigators. A US laboratory confirmed the material contained uranium and weapons-grade plutonium.
Prosecutors also allege that Mr Ebisawa sought to acquire large quantities of military-grade weapons on behalf of an unspecified rebel group in Myanmar.
The weapons included surface-to-air missiles, assault and sniper rifles, machine guns, rockets of various calibres and a variety of tactical gear.
"It is chilling to imagine the consequences had these efforts succeeded, and the justice department will hold accountable those who traffic in these materials and threaten US national security and international stability," assistant attorney general Matthew G Olden said in a statement on Wednesday.
In February 2020, Mr Ebisawa allegedly contacted the DEA agent about selling nuclear materials. According to US prosecutors, he explained via encrypted communications that uranium is "not good for your health".
In September that year, Mr Ebisawa allegedly emailed the undercover DEA agent a letter bearing the name of a mining company. He offered to sell 50 tonnes of uranium and thorium for $6.85m (£5.4m).
Prosecutors also say he sent photographs showing "a dark rocky material" with a Geiger counter, which is used to measure levels of radiation.
Mr Ebisawa faces charges including conspiracy to commit international trafficking of nuclear materials, narcotics importation conspiracy, conspiracy to acquire, transfer and possess anti-aircraft missiles and money laundering.
His co-conspirator in the case - 61-year-old Thai national Somphop Singhasiri - is facing drugs and weapons charges.
Both are facing life in prison if convicted.
The pair will be arraigned in a New York federal courtroom on Thursday.
