Washington (CNN) Former Secretary of State John Kerry has been engaging in shadow diplomacy to try to preserve the Iran nuclear deal, a major diplomatic achievement of his, according to a new report.

Over recent months, Kerry has been holding meetings and speaking with big players in the Iran nuclear agreement, who, like Kerry, do not want President Donald Trump to withdraw the US from the deal, The Boston Globe reported.

Trump condemned Kerry's reported engagement on Monday.

"The United States does not need John Kerry's possibly illegal Shadow Diplomacy on the very badly negotiated Iran Deal. He was the one that created this MESS in the first place!" Trump tweeted.

The United States does not need John Kerry's possibly illegal Shadow Diplomacy on the very badly negotiated Iran Deal. He was the one that created this MESS in the first place! — Donald J. Trump (@realDonaldTrump) May 7, 2018

Citing a person briefed on the meetings, the Globe reported that Kerry had met with Iran 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 limiting Iran's nuclear weapons program intact.

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