He claimed the Iranian regime supports terrorism and exports violence and chaos across the Middle East.
Earlier that day President Trump met with top defense officials to discuss the threat from North Korea and preventing Iran from getting a nuclear weapon.
The administration was considering October 12 for Trump to give a speech on Iran but no final decision had been made, an official said previously.
Trump had in the past called the Iran nuclear pact an "embarrassment" and "the worst deal ever negotiated".
A unilateral USA exit from the agreement would likely have undesirable consequences for Washington - isolating it from its negotiating partners that include close European allies, and potentially forcing Washington to sanction them if they continue to keep with the accord's parameters.
White House spokeswoman Sarah Sanders told reporters in Washington on October 5 that Trump had made "a decision" on the matter, adding that his primary focus "has been a comprehensive strategy on how to deal with Iran".
Nursery guard burns six children to death
The guard, who was aged about 50 and named as Damião Soares dos Santos, was badly burnt and later died of his injuries. The day-care center in Janauba, in the state of Minas Gerais , cares for about 70 children, according to local media.
Britain ready to walk away with no deal on Brexit, says Minister
Juncker's statement revealed a growing frustration within the European Union regarding the lack of clarity in the British position on Brexit .
Four killed in attack on Indian Kashmir paramilitary camp
Kashmir rebels have battled Indian forces for decades, demanding either independence or a merger with Pakistan. After the attack on Tuesday, the police said they were looking for seven more militants from the same group.
In his remarks to senior military leaders before the dinner, Trump outlined the challenges facing the United States.
If the deal is decertified, Congress would have 60 days to review the deal and make changes.
"You guys know what this represents", Trump asked the press as he pointed around the room, another journalist wrote of the incident.
Reporters asked whether he was referring to Iran or ISIS, but Mr Trump left them guessing, replying: "You'll find out".
Mattis suggested not leaving the agreement on Iranian nuclear program, since there is now no evidence of violations by Tehran and it meets the interests of Washington.
Donald Trump has cryptically warned that we are living in the "calm before the storm" - but has refused to explain what that means. Both were discussed at the talks in Thursday with the USA military representatives.