Yakuza leader Takeshi Ebisawa and co-defendant Somphop Singhasiri had previously been charged in April 2022 with drug trafficking and firearms offenses, and both were remanded.

He was then additionally charged in February 2024 with conspiring to sell weapons-grade nuclear material and lethal narcotics from Myanmar, and to purchase military weaponry on behalf of an armed insurgent group, prosecutors said.

The military weaponry to be part of the arms deal included surface-to-air missiles, the indictment alleged.

"As he admitted in federal court today, Takeshi Ebisawa brazenly trafficked nuclear material, including weapons-grade plutonium, out of Burma," said Acting US attorney Edward Kim, using another name for Myanmar.

"At the same time, he worked to send massive quantities of heroin and methamphetamine to the United States in exchange for heavy-duty weaponry such as surface-to-air missiles to be used on battlefields in Burma."

Prosecutors alleged that Ebisawa, 60, "brazenly" moved material containing uranium and weapons-grade plutonium, alongside drugs, from Myanmar.

From 2020, Ebisawa boasted to an undercover officer he had access to large quantities of nuclear materials that he sought to sell, providing photographs of materials alongside Geiger counters registering radiation.

During a sting operation including undercover agents, Thai authorities assisted US investigators in seizing two powdery yellow substances that the defendant described as "yellowcake."

"The (US) laboratory determined that the isotope composition of the plutonium found in the Nuclear Samples is weapons-grade, meaning that the plutonium, if produced in sufficient quantities, would be suitable for use in a nuclear weapon," the Justice Department said in its statement at the time.

One of Ebisawa's co-conspirators claimed they "had available more than 2,000 kilograms (4,400 pounds) of Thorium-232 and more than 100 kilograms of uranium in the compound U3O8 -- referring to a compound of uranium commonly found in the uranium concentrate powder known as 'yellowcake'."

The indictment claimed Ebisawa had suggested using the proceeds of the sale of nuclear material to fund weapons purchases on behalf of an unnamed ethnic insurgent group in Myanmar.

Ebisawa faces up to 20 years imprisonment for the trafficking of nuclear materials internationally.

Prosecutors describe Ebisawa as a "leader of the Yakuza organized crime syndicate, a highly organized, transnational Japanese criminal network that operates around the world (and whose) criminal activities have included large-scale narcotics and weapons trafficking."

Sentencing will be determined by the judge in the case at a later date, prosecutors said.