TEHRAN, Iran (AP) — Iran’s enriched uranium stockpile has passed the 300-kilogram limit set by the 2015 nuclear deal with world powers, the semi-official Fars news agency reported Monday.

The agency attributed its report to an unnamed source, who said U.N. inspectors had recently weighed Iran’s stockpile of low-enriched uranium. Iran had recently quadrupled its production of low-enriched uranium to be on pace to break one of the deal’s terms.

Iran has vowed to begin enriching its stockpile of uranium to higher levels closer to weapons grade later this month if world powers fail to negotiate new terms for the nuclear accord following the U.S. decision to withdraw from the agreement and restore crippling sanctions. European countries opposed the U.S. withdrawal and have repeatedly urged Iran to abide by the deal.

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The unraveling of the landmark nuclear agreement comes amid heightened tensions in the Persian Gulf, where Iranian forces shot down a U.S. drone last month. The U.S. said the drone was in international airspace while Iran insisted it veered across its border.

The crisis stems from President Donald Trump’s withdrawal from the nuclear accord and his policy of exerting maximum pressure on Iran to force it to change its policies in the region.

Iran’s Foreign Minister Mohammad Javad Zarif reiterated that his country would not bow to foreign pressure, the state-run IRNA news agency reported Monday.

“Today, Iran has to stand against U.S. economic sanctions through domestic production and relying on national potentials,” he said, without making any reference to the nuclear program.