The U.S. Navy's top official is warning of a "period of uncertainty" in the Persian Gulf following President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's decision to withdraw the U.S. from the Iran nuclear deal.

According to Reuters, Adm. John Richardson, the chief of Naval operations, said on Monday that U.S. forces are on heightened alert following Trump's announcement.

He added that the Pentagon is closely monitoring Iran's behavior.

Trump declared last week that the U.S. would pull out of the 2015 deal, which sought to curb Iran's pursuit of a nuclear arsenal in exchange for sanctions relief for the Islamic Republic of Iran.

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In making the announcement, Trump bucked the pleas of U.S. allies, particularly in Europe, who had urged in the president in recent weeks to remain in the deal.

They warned that a U.S. withdrawal from the pact could drastically raise tensions in the Middle East and accelerate Iran's pursuit of nuclear weapons. Iran has said that Europe has a "very limited opportunity" to preserve the deal.

Trump had threatened for more than a year to pull out of the deal, which he cast as a diplomatic disaster that failed to crack down on Iran's activities beyond its nuclear pursuits, such as its development and testing of ballistic missiles and its support for groups deemed terrorist organizations by the U.S.

Backers of the pact, however, argued that it addressed, at least temporarily, the most nefarious threat from Tehran: a nuclear weapons program.