Reuters reports: Samples taken by the U.N. nuclear watchdog at what Israel’s prime minister called a “secret atomic warehouse” in Tehran showed traces of uranium that Iran has yet to explain, two diplomats who follow the agency’s inspections work closely say.

The International Atomic Energy Agency (IAEA) is investigating the particles’ origin and has asked Iran to explain the traces. But Tehran has not done so, according to the diplomats, stoking tensions between Washington and Tehran. U.S. sanctions have slashed Iranian oil sales and Iran has responded by breaching its 2015 nuclear deal with world powers.

In a speech a year ago Israeli Prime Minister Benjamin Netanyahu, who vehemently opposed the deal, called on the IAEA to visit the site immediately, saying it had housed 15 kg (33 lb) of unspecified radioactive material that had since been removed.

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