President Trump tweeted Monday that he would announce a decision Tuesday at 2 p.m. ET on whether the U.S. will remain in the 2015 nuclear deal.

The president had given himself a May 12 deadline over the deal, which he has long criticized. Trump has previously signaled that he will pull out of the agreement, but has faced intense pressure from European allies not to do so.

Earlier Monday, Trump hit out at former Secretary of State John Kerry amid reports that Kerry had met with Iran Foreign Minister Javad Zarif as part of a bid to salvage the nuclear deal.

TRUMP RIPS KERRY FOR 'POSSIBLY ILLEGAL SHADOW DIPLOMACY ON IRAN DEAL

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

Kerry helped negotiate the agreement with Iran along with representatives of Great Britain, France, Germany, Russia and China. His meeting with Zarif was reported by the Boston Globe, which also claimed that the former Massachusetts senator had met with French President Emmanuel Macron; German President Frank-Walter Steinmeier, who was foreign minister of Germany when the deal was negotiated; and European Union foreign affairs chief Federica Mogherini.

British Foreign Secretary Boris Johnson wrote in a New York Times op-ed Sunday that the agreement offered the fewest disadvantages of all the options available. "It has weaknesses, certainly, but I am convinced they can be remedied. Indeed at this moment Britain is working alongside the Trump administration and our French and German allies to ensure that they are," he wrote.

In an interview with "Fox & Friends" Monday morning, Johnson conceded that Trump was "right to see flaws" in the agreement, but argued that those flaws could be fixed "without just throwing the baby out with the bathwater, without scraping the whole thing."

"If you do that, you have to ask the question of what next? What if the Iranians do rush for a nuclear weapon?" Johnson asked. "Are we seriously saying that we’re going to bomb those facilities or do we work with what we’ve got and push back on Iran together?"

Iran's president warned Trump on Sunday that leaving the nuclear deal would be a "historic regret."

House Armed Services Committee chairman Mac Thornberry, R-Texas, also advised against pulling out of the accord without a clearer idea of the consequences in an interview with "Fox News Sunday."

"Maybe the best thing is for the president to delay a bit more his deadline of this month and put the French and the British up to the test about whether it is possible to get this other sort of agreement," Thornberry said.

The Associated Press contributed to this report.