US President Barack Obama praised an agreement struck to curb Iran's nuclear programme over six months and argued that imposing additional US sanctions could scupper the deal.
"Now is the time to give diplomacy a chance to succeed," Obama said after the European Union revealed that Iran and six major powers had reached an accord to implement a November 24 nuclear agreement with Iran.
That agreement is designed to curtail Iran's nuclear activities for a six-month period beginning on January 20 in exchange for sanctions relief from the six major powers: Britain, China, France, Germany, Russia and the United States. Iran has agreed to halt high-grade uranium enrichment and neutralise its existing stocks of nuclear fuel that are near weapons grade.
Obama also urged the US Congress not to impose additional sanctions on Iran, saying that doing so risked undermining the November agreement, known as the Joint Plan of Action (JPA), which aims to give the two sides six months to reach a comprehensive deal to address all questions about whether Iran seeks nuclear arms.
Iran denies this, saying its programme is solely for peaceful purposes, such as generating electricity and medical isotopes.
"Imposing additional sanctions now will only risk derailing our efforts to resolve this issue peacefully, and I will veto any legislation enacting new sanctions during the negotiation," Obama said, echoing a previous veto threat he had made.
Obama said he had "no illusions about how hard it will be" to get a comprehensive agreement with Iran but that it was vital "for the sake of our national security and the peace and security of the world" to try to find a diplomatic solution.
The United States and Israel have both refused to rule out the possibility of military action against Iran's nuclear programme if the matter cannot be resolved diplomatically.
Separately, senior US officials for the first time offered details on how the estimated US$7 billion in sanctions relief envisaged in the November agreement would be implemented.
The officials said some sanctions relief would start on the first day of the six-month agreement's implementation and some withheld until its final day in July.
Assuming the International Atomic Energy Agency, the UN nuclear watchdog, confirms Iran is carrying out the deal, the major powers would immediately suspend sanctions on Iran's petrochemical exports, imports for its car manufacturing sector and on its trade in gold and other precious metals.
Of the estimated US$7 billion in sanctions relief over the six months, US$4.2 billion is in the form of access to currently blocked Iranian revenues held abroad. Other sanctions relief includes a pause on efforts to reduce further Iran's exports of crude oil.