A landmark deal that curbs parts of Iran's disputed nuclear programme in exchange for sanctions relief will take effect from a January 20 target date, Tehran and Washington said Sunday.
US President Barack Obama welcomed the news but warned there was still a rough road ahead before a comprehensive solution can be struck.
Western nations and Israel have long suspected Iran of pursuing a nuclear weapons capability alongside its civilian programme, charges the Islamic republic fiercely denies.
Tehran agreed in November to roll back parts of its nuclear work and halt further advances in exchange for the release of billions of dollars in frozen assets and limited relief from sanctions that have choked its hard-hit economy.
The deal was a major achievement for President Hassan Rouhani, who won a first-round electoral victory over a pool of conservatives last year by vowing a more diplomatic approach with the West after eight years of stalled talks and escalating sanctions under his hardline predecessor, Mahmoud Ahmadinejad.
"Both sides reached the same interpretation on how to implement the agreement and the first step will be executed from January 20," Iran's chief nuclear negotiator Abbas Araqchi said, quoted by IRNA news agency.
The White House was quick to confirm the news.
"Beginning January 20th, Iran will for the first time start eliminating its stockpile of higher levels of enriched uranium and dismantling some of the infrastructure that makes such enrichment possible," it said.
Obama hailed the agreement as an "important step forward" while stressing the focus was now "on the critical work of pursuing a comprehensive resolution that addresses our concerns over Iran's nuclear programme".
"I have no illusions about how hard it will be to achieve this objective, but for the sake of our national security and the peace and security of the world, now is the time to give diplomacy a chance to succeed," said Obama.
Obama warned against Congress imposing any new sanctions on Iran during the next phase of negotiations, saying he would veto any such move.
British Foreign Secretary William Hague and his US counterpart John Kerry also welcomed the setting of the implementation date.
"The entry into force of this agreement on 20 January is an important step towards peacefully resolving the Iranian nuclear issue, on which comprehensive negotiations will now start," said Hague.
Kerry said the development was a "critical, significant step forward".
After two days of exhaustive talks, Iran and the European Union -- which represents the P5+1 of Britain, China, France, Russia and the United States plus Germany -- agreed Friday on how to implement the deal.
EU foreign policy chief Catherine Ashton said the agreement lays "the foundations for a coherent, robust and smooth implementation" of the November deal for the next six months.
New generation nuclear centrifuges
Iran and world powers have held several sessions of talks in Vienna and Geneva to fine-tune the deal in the past weeks and the date of January 20 was reached at the last round in the Swiss city.
Diplomats said there were three main hurdles at the last session of negotiations, namely over a new generation of Iranian nuclear centrifuges which could potentially enable Tehran to purify uranium to a weapons-grade level.
Iran's nuclear chief Ali Akbar Salehi defended Tehran's "right" to carry out reasearch on advanced centrifuges.
In August, Iran said it has about 19,000 centrifuges, including 1,000 of new P-2 generation, confirming figures from the UN watchdog overseeing its nuclear drive.
Among the main points of the November deal, Iran agreed it will not enrich uranium over five percent for the duration of the six months and committed to neutralise its entire stockpile of uranium enriched to 20 percent.
It also agreed to allow daily site inspections of its Fordo and Natanz enrichment facilities by experts from the UN nuclear watchdog IAEA as well as hand over information about the design of the Arak reactor.
Araqchi said on Sunday that in return, Iranian assets frozen abroad would be gradually released.
"All the issues agreed upon will be executed on the same day (January 20) except one: from their side about the payment of $4.2 billion of Iran assets blocked outside of the country; from our side to turn (half) 20% enriched uranium into oxide or diluting it," he said, quoted by ISNA news agency.
"These two things will take place within the six month period."