(CNN) Former US Secretary of State John Kerry urged US President Donald Trump not to pull out of the Iran nuclear deal on Tuesday, warning that withdrawing from the pact could lead to conflict and further destabilize the Middle East.

Trump is due to make an announcement Tuesday on whether the US will quit the deal to curtail Iran's nuclear program, which was signed in 2015 and is one of the signature diplomatic achievements by Kerry and the Obama administration.

Trump, who has repeatedly criticized the deal, is expected to say he will allow sanctions to go forward on Iran, a first step toward withdrawing from the pact, according to a US official and a person familiar with the plan. The officials cautioned that nothing is final until Trump makes his announcement later Tuesday.

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 "all Americans should care enormously about not igniting new conflict, not creating a more threatening situation that we have today."

"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 also responded to reports that he has been working behind the scenes on the Iran deal, much to Trump's perceived annoyance.

Kerry met with Iranian Foreign Minister Javad Zarif at the United Nations in New York two weeks ago, their second meeting in about two months, to discuss ways of keeping the deal, according to two sources familiar with the interactions.

"I am an ex-secretary of state who has great friendships and great relationships with people around the world," Kerry said Tuesday. "I travel around the world for business and other things and I meet with people and have conversations and I stay current. That is a normal process for any ex-secretary or anyone who has been involved in public affairs."

"I am not engaged in any particular strategy or anything, but I certainly think the President should stay in the agreement. I think that America is stronger if we stay in the agreement."

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.

Iran, UK react ahead of Trump's announcement

Speaking from Tehran hours before Trump's announcement, Iranian President Hassan Rouhani insisted his country was after "constructive relations" with the world, the state-run Islamic Republic News Agency (IRNA) reported.

"From the beginning, the basic work of the government has been to interact correctly (with other countries) and implement win-win policies," Rouhani said. "However, special factors, such as a certain man becoming president in a country, could create special conditions which may be negative.

"But we shall overcome them. It may take two to three months for us to overcome the problems, but we will overcome them."

While all eyes in Tehran will be on Washington, Europe will also be closely watching Trump's remarks.

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European leaders have repeatedly urged Trump not to leave the Iran deal. Macron, the French President, and German Chancellor Angela Merkel both traveled to Washington in recent weeks in a seemingly unsuccessful bid to change Trump's mind.

On Tuesday, UK Prime Minister Theresa May said she believed it was "right to maintain the Iran nuclear deal as the best way of neutralizing the threat of a nuclear-armed Iran."

"We'll continue working closely with the US and our EU partners on how to address the range of challenges Iran poses, including those issues which a new deal might cover," a Downing Street spokesperson said.

Russia has also warned against the potential of "harmful consequences" of scrapping the deal.

On Tuesday, Dmitry Peskov, spokesman for Russian President Vladimir Putin, said the possible consequences would threaten the end of a "fragile but irreplaceable system of agreements between the involved parties regarding the Iranian nuclear dossier and non-proliferation in general."