Iran nuclear deal 'not legally binding': US
The deal, which saw world powers lift sanctions on Tehran in return for controls on its nuclear programme last year, could be dropped if a US administration decided to walk away.
"Any party - and I'm speaking very hypothetically here, because I don't want in any way to attempt to hypothesise about what the incoming administration's going to do - I'm just talking purely about an agreement that any party can walk away from," State Department spokesman Mark Toner said.
"And that will have profound consequences on the integrity of the agreement."
The outgoing US Obama administration – which peddled the agreement – is proud of the historic move and has made no moves to suggest it would drop the deal, but Trump said several times during his campaign that moving away from it will be a priority once he takes power in January.
On Wednesday, Iran's President Hassan Rouhani argued the deal has been enshrined in international law.
"Iran's understanding in the nuclear deal was that the accord was not concluded with one country or government but was approved by a resolution of the UN Security Council and there is no possibility that it can be changed by a single government,” he said.
However according to Toner, the Iran deal was not a legally binding treaty, but that the current US administration believes it is in Washington's interest to continue it.
Asked whether if the US withdraws support for the deal whether Iran might start building a nuclear weapons program, Toner said: "Yes. That's the reality of the situation."
"This is why we believe it to be in everyone's interest, including the world's interest, that if Iran abides by this agreement, and all the parties abide by the agreement, then we have shut-off Iran's path to a nuclear weapon," he said.
During the election campaign, Trump described the deal as "disastrous" and said it would be his "number one priority" to dismantle it.
After the deal came into force in January, Iran reduced by two-thirds the number of its uranium enrichment centrifuges, slashed its uranium stockpile and removed the core of the Arak reactor.
Despite the lifting of sanctions, the United States and the European Union still impose penalties against Iran over its support for armed movements, ballistic missile programme and human rights record.
Iran provides financial and military support to Hizballah and Palestinian Islamist groups, considered by some Western countries as terrorist organisations.