By Arshad Mohammed and Roberta Rampton
WASHINGTON (Reuters) - U.S. President Barack Obama hailed an agreement struck on Sunday to curb Iran's nuclear program over six months and argued that imposing additional U.S. sanctions could scupper the deal.
"Now is the time to give diplomacy a chance to succeed," Obama said in a written statement after the European Union said 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.
Obama also urged the U.S. Congress not to impose additional sanctions on Iran, saying that doing so risked undermining the November 24 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 program is for solely 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 in the statement, echoing a previous veto threat.
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 program if the matter cannot be resolved diplomatically.
FIRST $550 MILLION PAYMENT AROUND FEB. 1
Separately, senior U.S. officials for the first time offered details on how the estimated $7 billion in sanctions relief envisaged in the November 24 agreement will be meted out.
The officials, who spoke to reporters on condition that they not be identified, said that some sanctions relief will start on the first day of the six-month agreement's implementation - January 20 - and some withheld until its final day.
Assuming the International Atomic Energy Agency (IAEA), the U.N. nuclear watchdog, confirms Iran is carrying out the deal, the major powers would immediately suspend sanctions on Iran's petrochemical exports, imports for its auto manufacturing sector and on its trade in gold and other precious metals.
Of the estimated $7 billion in sanctions relief over the six months, $4.2 billion is in the form of access to currently blocked Iranian revenues held abroad.
One official said access to some of these funds depended on Iran keeping its commitment to dilute half of its 20 percent enriched uranium to no more than 5 percent enriched uranium.
Another official said the first $550 tranche would be paid on or about February 1, and the final payment, of the same amount, on or about July 20th. A total of $900 million would depend on Iran diluting the enriched uranium, this official said.
Other sanctions relief includes a pause on efforts to reduce further Iran's exports of crude oil, which have been cut to about 1 million barrels per day from about 2.5 million at the start of 2012, before U.S. and European sanctions took effect.
The United States would also move quickly on applications for licenses to provide Iran with aircraft parts and services to ensure the safety of its civilian aircraft and to establish a financial channel to facilitate humanitarian trade.
The Obama administration has urged Congress for weeks not to pass additional sanctions legislation on Iran.
Sanctions proponents argue that the economic penalties have brought Iran to the negotiating table and that keeping up the pressure with more sanctions is the way to ensure that Iran keeps to the November 24 accord and negotiates a comprehensive deal.
Fifty-nine of 100 U.S. senators have signed on to a bill that would require further cuts in Iran's oil exports, including 16 Democrats.
If the bill were brought up for a vote, it would need at least 60 votes to pass. Senate Majority Leader Harry Reid has not yet indicated when - or whether - he would allow a vote on the measure.
If Obama vetoed sanctions passed by Congress, at least two-thirds of the members in each of the Senate and the 435-member House of Representatives would have to vote in favor of overriding the veto.
The House overwhelmingly passed a bill calling for tougher sanctions in July, months before the nuclear talks started.
(Additional reporting by Patricia Zengerle; Writing by Arshad Mohammed and Roberta Rampton.; Editing by Leslie Adler, Rosalind Russell and Meredith Mazzilli)