Donald Trump tweeted he would announce his Iran deal decision this morning. AP

US President Donald Trump was expected to tell the world whether he planned to follow through on his threat to pull out of the landmark nuclear accord with Iran that would almost surely ensure its collapse.

There were no signs that European allies enlisted to "fix" the deal had persuaded him to preserve it.

In a burst of last-minute diplomacy, punctuated by a visit by Britain's top diplomat, the deal's European members gave in to many of Trump's demands, according to diplomats, officials and others briefed on the talks.

Yet they still left convinced he was likely to reimpose sanctions and walk away from the deal he had lambasted since his days as a presidential candidate.

Hanging in the balance was the fate of the 2015 nuclear deal struck by the United States, Iran and world powers that lifted most US and international sanctions against the country. In return, Iran agreed to restrictions on its nuclear program making it impossible to produce a bomb, along with rigorous inspections.

Building up anticipation for the big reveal, Trump announced on Twitter he would disclose his decision at 2pm - 2am in Hong Kong - with an announcement set for the Diplomatic Room of the White House.

With uncharacteristic discipline, he kept the decision confined to a small group within the White House National Security Council, leaving even many of his aides guessing what he had decided.

If the deal is abandoned, Iran would be free to resume prohibited enrichment activities, while entities doing business with Iran would have to scramble to extricate themselves or run afoul of the United States. As they braced for an expected withdrawal, US officials were dusting off plans for how to sell a pullout to the public and explain its complex financial ramifications, said the diplomats, officials and others.

There was at least as much guessing in Iran, where many were deeply concerned about how Trump's decision could affect the already struggling economy.

In Tehran, President Hassan Rouhani sought to calm nerves, smiling as he appeared at a petroleum expo. He did not name Trump directly, but emphasized that Iran continued to seek "engagement with the world."

He added: "It is possible that we will face some problems for two or three months, but we will pass through this."

An immense web of sanctions, written agreements and staggered deadlines make up the accord.

So Trump effectively has several pathways to pull the United States out of the deal by reneging on its commitments.

Under the most likely scenario, Trump would allow sanctions on Iran's central bank - intended to target its oil exports -to kick back in, rather than waiving them once again on Saturday, the next deadline for renewal.

ASSOCIATED PRESS