In its first major violation of the 2015 nuclear deal, Iran announced that it has enriched uranium beyond the limits of the agreement.

Iranian President Hassan Rouhani reportedly carried through on his threat to increase Iran’s percent concentration of uranium-235 beyond the 3.67% threshold set forth. The level was set in the 2015 nuclear deal, which the U.S. withdrew from last year.

Behrouz Kamalvandi, spokesman for Iran’s Atomic Energy Organization, said that Iran had passed 4.5% enrichment over the weekend. Kamalvandi also threatened to restart a dismantled centrifuge or enrich uranium to 20% or more. Around 90% enrichment is typical for nuclear weapons.

President Trump warned Iran last week about breaching the 3.67% limit, saying that threats to increase the concentration of fissile material would come back to bite the country like “nobody has been bitten before.”

The move by Tehran comes amid simmering tensions between the two powers. The U.S. had planned airstrikes on Iranian facilities in response to the country’s downing of a U.S. drone, but Trump called off the move at the last moment.

Last month, the U.S. imposed new sanctions against Iran, targeting its leader and top officials. The U.S. also announced that it was sending an additional 1,000 troops to the Middle East after a pair of oil tankers were attacked in the Gulf of Oman.