German Chancellor Angela Merkel, standing alongside US President Donald Trump at the White House, said today that the existing international nuclear deal with Iran is not enough to curb the Islamic republic's ambitions in the region.

The so-called Joint Comprehensive Plan of Action, signed by Tehran and six world powers including Germany and the United States, is "a first step that has contributed to slowing down their activities in this particular respect," Ms Merkel told reporters.

"But we also think from a German perspective that this is not sufficient in order to see to it that Iran's ambitions are curbed and contained."

"Europe and the United States ought to be in lock step on this," she said.

Ms Merkel and Mr Trump put on a display of warmth and friendship during a White House meeting earlier today despite differences over trade and Iran that have sparked tensions between the two allies.

After their last White House meeting drew attention when the two leaders did not shake hands in the Oval Office, Mr Trump made a point of doing just that, twice, while congratulating the German chancellor on her election win.

"We have a really great relationship, and we actually have had a great relationship right from the beginning, but some people didn't understand that," Mr Trump said, calling Ms Merkel a "very extraordinary woman".

Ms Merkel acknowledged that it took a while to form a government after heavy election losses to the far-right, but she said it was important to her to make her first trip out of Europe since establishing her administration to Washington.

The cautious Ms Merkel has not established a particularly strong personal rapport with the brash Mr Trump, and the mood of her one-day working visit contrasted sharply with the tactile "bromance" between Mr Trump and French President Emmanuel Macron.

Earlier this week Mr Macron called on the United States not to abandon the Iran deal as western envoys said Britain, France and Germany were nearing agreeing a package they hope could persuade Mr Trump to save the pact. 

Mr Trump will decide by 12 May whether to revive US sanctions on Iran. 

Doing so would be a serious blow to the nuclear deal, which many western countries sees as essential for stopping Tehran developing a nuclear bomb.