But he said he had given "the necessary orders", notably to Iran's Atomic Energy Organisation, in anticipation of Trump's decision.
After Trump's tweet on Monday morning, a Kerry spokesman provided a statement defending Kerry's efforts.
"Today we are telling the world that if you are anxious about Iran's access to nuclear bomb, we have completely allayed this concern in the JCPOA and the deal ensures that Iran does not seek nuclear weapons".
He will not meet President Trump, but is expected to appear on the Fox & Friends morning news, which Mr Trump is known to watch avidly.
The U.N. nuclear watchdog says it is maintaining the "world's most robust verification regime" in Iran and has repeatedly said Tehran is complying with the deal terms. We haven't got there yet.
At the same time, the Europeans, having convinced Russian Federation and China to back their strategy, approached the U.S. with another stark choice: either join an worldwide coalition to apply diplomatic pressure on Iran, or pursue dubiously effective military measures on your own. "We've got to be tougher'".
It is not immediately clear what Trump will announce Tuesday or whether he will announce the end of the deal or push for a renegotiation.
Kerry met privately two weeks ago at the United Nations with Iranian Foreign Minister Javad Zarif to discuss ways to preserve the pact, which is supposed to prevent Iran from developing nuclear weapons in exchange for easing economic sanctions.
"I say that a deal that enables Iran to keep and hide all its nuclear weapons know-how, is a disgusting deal", he said. Johnson asked: "Are we seriously saying that we will bomb those facilities ... is that really a realistic possibility?"
Johnson's trip follows similar endeavors by German Chancellor Angela Merkel and French President Emmanuel Macron, who both traveled to the US late last month and attempted to persuade Trump not to leave the Iran pact.
Mr Johnson is on a two-day trip to Washington to meet with Mike Pence, the Vice President, and other senior USA officials to persuade them not not back out of the deal. If they go back into effect, the US will nearly certainly be in violation of the agreement.
He also said that one of the bitterest facts of the world is that the Saudi-led coalition is bombarding the oppressed people of Yemen. It's already a very, very risky state at the moment, we don't want to go down that road. He also seems to be hopeful that the threat of military action, either US or Israeli, will deter Iran from resuming its nuclear activities. However, Israeli Prime Minister Benjamin Netanyahu, last week, pointed out that the deal, which was a significant achievement of President Barack Obama's presidency, was "fatally flawed".
Writing in the New York Times, Johnson argued "only Iran would gain" from abandoning nuclear restrictions.
Mr Trump has signalled he would pull out of the agreement by the deadline unless it was revised, but faces intense pressure from European allies not to do so. He heads to Moscow later this week for a meeting with President Vladimir Putin, where talks will focus on the Iranian nuclear program and Iran's involvement in neighboring Syria. "If all Kerry is doing is working to keep in place something that's still technically a 'measure of the United States, ' I don't see how the statute would apply even if someone was insane enough to try it".
To Iran, European diplomats offered a choice between two futures: one in which Iran would freeze its nuclear program and end its worldwide isolation; and one in which it would maintain its program and face ever-harsher sanctions, and possibly war.
Iranian President Hassan Rouhani's government says the U.S.is "bullying" his country.
He also appealed to Trump's pride in progress made with a nuclear-armed state, North Korea.