Iran announced Monday that it has exceeded the limit on its stockpile of low-enriched uranium it agreed to in the 2015 international accord restraining its nuclear weapons development.

Foreign Minister Mohammad Javad Zarif told the semi-official news agency ISNA, “Iran has crossed the 300-kilogram limit …,” which curtailed its stockpile of uranium enriched up to 3.67%.

The United Nations atomic watchdog agency confirmed Monday that Iran had exceeded the limits imposed by the deal.

Iran’s announcement is believed to be the first time that Iran has broken one of the provisions of the pact it agreed to with six world powers, including the United States, and the European Union in exchange for lifting of economic sanctions against Tehran.  The U.S. withdrew from the accord last year. 

Israeli Prime Minister Benjamin Netanyahu said Monday Iran’s move is a “significant step toward making a nuclear weapon” and urged Europe to impose “automatic sanctions” on Iran.

Britain’s Foreign Secretary Jeremy Hunt said on Twitter he was “deeply worried” by Iran’s actions and urged Tehran “to avoid any further steps away from JCPoA and come back into compliance.”

Russia’s Deputy Foreign Minister Sergei Ryabkov said the development “causes regret, but shouldn’t be overdramatized.” He said the action is a “nature consequence of recent events” and the result of the United States’ “unprecedented pressure” campaign on Iran. 

U.S. President Donald Trump contended the nucear deal would not keep Iran from developing a nuclear weapon. Trump instead reimposed stiff new sanctions that have hobbled the Iranian economy, chiefly aimed at curbing Iran’s international oil exports, and is aimed at forcing Iran into new negotiations.

Iran’s breach of the 2015 pact comes at a time of heightened tension in the Middle East centered on Tehran, whose military two weeks ago shot down an unmanned U.S. drone, with Washington claiming it did so in international airspace while Iran says it was over its territory. The U.S. and Israel have also blamed Tehran for attacks on oil tankers in the Persian Gulf, on oil fields in Saudi Arabia and in Baghdad.

Some U.S. national security advisers urged Trump to attack Iran military targets after the drone strike, but he backed off at the last minute after learning that such an attack would kill about 150 Iranians, which he said he did not think was a proportionate response to the downing of the unmanned drone.