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.

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.

The former secretary of state also met last month with German President Frank-Walter Steinmeier, separately sat down with French President Emmanuel Macron in both Paris and New York, and spoke on the phone with European Union foreign policy chief Federica Mogherini, the source told the Globe.

Kerry has also quietly lobbied members of Congress, including House Speaker Paul Ryan, placing dozens of phone calls in recent weeks.

The White House did not respond to a request from the Globe for comment.

During his Friday speech to the National Rifle Association, Trump attacked Kerry for his initial role in negotiating the Iran deal, which he called “horrible.”

“And we have the former administration as represented by John Kerry, not the best negotiator we’ve ever seen,” Trump said. “He never walked away from the table, except to be in that bicycle race where he fell and broke his leg.”

The Trump administration faces a May 12 deadline to decide whether to continue waiving sanctions on Iran that were lifted under the 2015 Iran deal, formally known as the Joint Comprehensive Plan of Action.