Trump is due to make an announcement Tuesday on whether the US will quit the deal, which was signed in 2015 and is one of the signature diplomatic achievements by Kerry and the Obama administration.
Kerry has been a fierce advocate of the agreement, labeling it “the single toughest and most transparent” nuclear agreement in the world.
Speaking in Milan on Tuesday ahead of Trump’s announcement, Kerry said leaving the deal could lead to potential conflict.
“Our friends are safer if we stay in this agreement. We made an agreement, Iran is living by the agreement, yes we have concerns on the missiles and Yemen and other things, but we should be working on those,” Kerry told CNN.
“The Obama administration made a clear decision that working on those other issues, making progress on those, is easier with an Iran that does not have a nuclear weapon then with an Iran which is working towards one.”
Kerry added that “to pull out of it and try to focus on those things totally complicates the ability to achieve the goals, and I think it could actually lead to conflict potentially.”
Trump fired back at Kerry on Tuesday, warning him to “stay away from negotiations” around the Iran deal.
“John Kerry can’t get over the fact that he had his chance and blew it! Stay away from negotiations John, you are hurting your country!” Trump tweeted early Tuesday.
Trump had previously stated he would make a decision ahead of a May 12 deadline on whether or not to continue waiving US sanctions on Iran that were lifted under the deal, formally known as the Joint Comprehensive Plan of Action (JCPOA).
As part of the 2015 pact — agreed by Iran, the US, France, the United Kingdom, the European Union, Russia and China — Iran must reign in its nuclear program in return for international sanctions being lifted.
Trump has long been critical of the accord, last month describing it as “insane” and “ridiculous” during a state visit with French President Emmanuel Macron, a key supporter of the deal.