US President Donald Trump has warned Iran of "big problems" if it resumes the nuclear programme it agreed to curb in a 2015 international accord. Photo: GETTY IMAGES

Speaking in the White House Oval Office as he hosts French President Emmanuel Macron, Mr Trump called the Iran deal "a disaster" and "insane".

The US president has been threatening to reject an extension of the Obama-era nuclear pact by a 12 May deadline.

Mr Macron is in Washington lobbying Mr Trump to preserve the pact.

Germany's Chancellor Angela Merkel is due to visit Washington on Friday to make a last-minute bid to dissuade the president from potentially torpedoing the agreement.

"It won't be so easy for them to restart," Mr Trump said on Monday when a journalist asked him about the possibility of Iran relaunching their nuclear programme if the deal is scrapped.

"They're not going to be restarting anything. They restart it they're going to have big problems, bigger than they've ever had before.

"And you can mark it down - they restart their nuclear programme, they will have bigger problems than they've ever had before."

He added: "We're not going to allow certain things to happen that are happening. The Iran deal is a disaster. They're testing missiles. What's that all about?"

Mr Trump's stark warning comes a day after Iranian President Hassan Rouhani threatened "severe consequences" if the US withdraws from the nuclear deal.

Mr Rouhani did not specify what retaliatory action Tehran might take. But his Foreign Minister, Javad Zarif, has said a probable response would be to restart the enrichment of uranium - a key bomb-making ingredient.

Under the agreement, Tehran agreed to curb its nuclear programme, which it maintains was for peaceful civilian purposes, in return for an easing of economic sanctions.

Mr Trump has long complained that the deal - signed by the US, Iran, Europe, Russia and China - does nothing to halt Iran's support for militant groups in the region such as Hezbollah.

During the meeting, Mr Macron spoke in French and then translated into English, saying: "The Iran deal is an important issue but we have to take a far broader picture which is security in the overall region."

"What we want to do is to contain Iran and its presence in the region," he added.

At one point during the meeting, Mr Trump wiped away "dandruff" from Mr Macron's shoulder.

"We do have a very special relationship," Mr Trump told journalists gathered in his office.

"In fact I'll get that little piece of dandruff off.

"Little piece! We have to make him perfect. He is perfect."

Earlier in the day, Mr Macron and his wife were welcomed to the White House with a military ceremony and a 21-gun salute.

In welcoming remarks, Mr Trump thanked France for joining the US and Britain earlier this month in launching air strikes after an alleged chemical attack in Syria.