President Trump announced Wednesday that he will add to existing sanctions on Iran, after the Islamic republic admitted Monday that it surpassed uranium enrichment levels that were set by the Iran Nuclear Agreement in 2015. The U.S. backed out of that deal, but several European nations remain involved.

The deal said that Iran could not enrich uranium above 3.67 percent, and Behrouz Kamalvandi, a spokesman for Iran's Atomic Energy Organization, said the country has passed that and may proceed further, Reuters reported. The 3.67 percent level is enough for nuclear power reactors, but far short of the 90 percent needed for weapons.

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“Iran has long been secretly ‘enriching,’ in total violation of the terrible 150 Billion Dollar deal made by John Kerry and the Obama Administration. Remember, that deal was to expire in a short number of years,” Trump tweeted. “Sanctions will soon be increased, substantially!”

Iran's previous announcement that it would boost uranium enrichment above the cap prompted a rebuke from President Trump on Sunday, saying: “Iran better be careful. … Iran is doing a lot of bad things,” Trump said. “Iran will never have a nuclear weapon.”

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President Trump has already imposed significant sanctions against Iran as part of an effort to cripple its economy and force it back to the negotiating table to work out a new deal that imposes stricter requirements than the one arranged under the Obama administration.

On Tuesday, Sen. Marco Rubio, R-Fla., supported Trump’s tough stance against Iran, calling the country “a terroristic regime that needs to be reined in.”

Tension between the U.S. and Iran has escalated in recent months, as Iran shot down an unmanned American drone that was flying over international waters. The Trump administration has also accused Iran of attacking oil tankers in the same region in the Gulf of Oman, although the Iranian government did not claim responsibility.

Fox News’ Bradford Betz contributed to this report.