The landmark 2015 deal to curb Iran's nuclear programme is hanging in the balance, with Trump threatening to walk away from the accord unless it is expanded to include Iran's ballistic missile programme and regional activities by a May 12 deadline.

Trump slammed the deal as "insane" at the White House with Macron on Tuesday but said there was a "great shot" at a new accord, while Macron said he wanted to work for a new arrangement.

The EU's Federica Mogherini, who has staunchly defended the deal, said Wednesday that it was working and was "essential for European security" but appeared to leave the door open for some new arrangement.

"On what can happen in the future we'll see in the future, but there is one deal existing, it's working, it needs to be preserved," the former Italian foreign minister said as she arrived for a donor conference on Syria in Brussels.

Europe, led by Britain, France and Germany, has repeatedly tried to persuade Trump not to abandon the 2015 deal, which gave Iran massive sanctions relief and the guarantee of a civilian nuclear programme in return for limiting enrichment that could produce weapons grade fuel.

German Chancellor Angela Merkel will renew those calls when she follows Macron to Washington on Friday.