Washington (CNN) President Donald Trump will announce his decision on whether the US will pull out of the Iran nuclear accord at 2 p.m. ET on Tuesday, he wrote in a tweet.

"I will be announcing my decision on the Iran Deal tomorrow from the White House," he wrote Monday afternoon.

Trump is weighing whether to continue waiving sanctions on Iran's energy and banking sector that were lifted as part of the 2015 agreement in exchange for curbs on Tehran's nuclear program.

The President is widely expected to decide against extending the sanctions waivers, and in doing so, effectively leave the nuclear deal. Trump excoriated the agreement, even before winning the White House, as the "worst deal ever" and promised to tear it up on his first day in office.

One European diplomat said it seems fairly clear that the administration will walk away from the deal, and described the chances that the Joint Comprehensive Plan of Action, as the deal is formally known, will continue intact as "very small."

"It's pretty obvious to me that unless something changes in the next few days, I believe the President will not waive the sanctions," the diplomat told CNN. "And that will have various consequences that I think we have yet fully to understand and spell out."

The implications of a US departure from the agreement aren't clear yet, but analysts have warned that it would send a message to other nations -- particularly North Korea -- about the reliability of the US as a negotiating partner.

Other analysts have pointed to the potential for a Middle East arms race if Iran also leaves the deal and re-starts its nuclear program and say it will be even harder to confront Iran's regional activity if it does so. A decision to leave the deal also raises questions about the fate of at least four Americans held in Iran and the leverage Washington will have to help them.

Trump's supporters argue that leaving the pact is necessary in order to confront Iran's disruptive behavior in the region, including its support for Houthi rebels in Yemen, for the regime in Syria and for Hezbollah.

"If the President decides to leave the deal, it's hardly the end of the world -- or even bad," said Jim Phillips, a senior research fellow at the conservative Heritage Foundation. "Instead, it should be seen as a necessary step to confront one of the deadliest threats Americans, Israelis, and Gulf Arabs face."

Despite Trump's hostility toward the agreement, a signature achievement of the Obama administration, the International Atomic Energy Agency has repeatedly found that Iran is complying with its terms, and Europe, Russia and China have continued to support the deal.

Trump has raged that the agreement doesn't address Iran's ballistic missile program and its activities in the region and that certain parts of the agreement "sunset" after a few years.

European allies -- including the United Kingdom, France and Germany -- have encouraged Trump to remain part of the deal while acknowledging the agreement's flaws. US negotiators had been working with European counterparts to develop an ancillary agreement that would address Tehran's missile program and its support for radical groups in the Mideast.

'Iran will not renegotiate'

The European diplomat said the administration's intent seems to be to renegotiate a second deal on Iran's nuclear program while also working on agreements on missiles and "Iran's malign activity in the Middle East."

Iran's foreign minister has said Iran won't renegotiate. "Iran will not renegotiate what was agreed to years ago and has been implemented," Javad Zarif said in a video posted to YouTube on Thursday. "We will neither outsource our security nor will we renegotiate or add on to a deal we have already implemented in good faith."

French President Emmanuel Macron issued his own warning Sunday, telling German magazine Der Spiegel that if the US leaves the deal, "we would open the Pandora's box. There could be war." He added, "I don't think that Donald Trump wants war."

Macron has been one of a series of high-profile visitors to come to Washington in the last few weeks to argue for remaining in the deal and to outline the costs of leaving.

The French president and German Chancellor Angela Merkel made back-to-back visits in April, while British Foreign Secretary Boris Johnson spent Monday in Washington making media appearances and meeting with administration officials in an attempt to make a case for the deal.

Appearing on one of the President's favorite TV programs, Fox & Friends, Johnson asked rhetorically what the US plan would be if Trump decides to opt out. Many in Washington wonder about the possibility of a military strike against Tehran - particularly with the recent appointment of National Security Advisor John Bolton, a hawk on Iran.

"Are we seriously saying that we will bomb those facilities ... is that really a realistic possibility?" Johnson asked.

Johnson also warned in the interview about the consequences of scrapping the deal negotiated by the US, UK, France, Germany, China, Russia and the EU. It offered Iran nuclear-related sanctions relief in exchange for strict curbs on its nuclear program, which could no longer apply if Iran decides to leave the deal as well.

"If they do get a nuclear weapon, you're going to get an arms race in the Middle East," Johnson said. "You're going to have the Saudis wanting one, the Egyptians wanting one, the Emiratis. It's already a very, very dangerous state at the moment, we don't want to go down that road. There doesn't seem to me at the moment to be a viable military solution."