Shortly after the interim accord takes effect, an Iranian official added, Tehran and world powers will start negotiating a final settlement of their differences about activity the West suspects is aimed at obtaining a nuclear weapons capability.
Iran says its atomic energy programme is aimed purely at electricity generation and other civilian purposes, although past Iranian attempts to hide sensitive nuclear activity from U.N. non-proliferation inspectors raised concerns.
The Nov. 24 agreement appeared to halt a slide towards another, wider Middle East war over Iran's nuclear aspirations, but diplomats warn it will not be easy to carry out because of longstanding mutual mistrust.
The Iranian official, Deputy Foreign Minister Abbas Araqchi, said the deal would allow Iran to stop complying if it saw its partners not living up to their own commitments.
"We don't trust them," he told state television, reflecting ingrained suspicions between Iran and the West that underlie what have been protracted negotiations.
In Washington, President Barack Obama said the United States and other nations would begin to give Iran "modest relief" on economic sanctions as long as the Islamic Republic lived up to its end of the agreement.
Obama said he would veto any new sanctions passed by the U.S. Congress during talks on a long-term deal with Iran, but said Washington would be prepared to increase its sanctions if Iran fails to abide by the agreement.
"Capitals have confirmed the result of the talks in Geneva ... The Geneva deal will be implemented from Jan. 20," Marzieh Afkham of the Iranian Foreign Ministry told reporters in Tehran, the semi-official Mehr news agency said.
European Union foreign policy chief Catherine Ashton also confirmed the date, and said the sides would now ask the United Nations' nuclear watchdog to verify the deal's implementation.
"We will ask the IAEA to undertake the necessary nuclear-related monitoring and verification activities," she said in a statement, referring to the International Atomic Energy Agency.
SENSITIVE NUCLEAR WORK
Ashton represents the six powers - the United States, Russia, China, France, Britain and Germany - in contacts with Iran related to its contested nuclear programme.
Senior officials from the European Union and Iran met in Geneva on Thursday and Friday to iron out remaining practical questions related to the implementation of the Nov. 24 deal, under which Iran agreed to curb its most proliferation-sensitive nuclear activity - higher-level uranium enrichment - in return for some relief from Western economic sanctions.
Such relief would include suspension of some restrictions on trade in gold, precious metals and petrochemicals, and in the auto industry. The deal allows third-country purchases of Iranian oil to remain at current levels. Some $4.2 billion in oil revenues would be allowed to be transferred to Iran.
EU spokesman Michael Mann said on Friday that any agreements would need to be validated by the governments of Iran and the six powers.
The accord is designed to last six months and the parties hope to use the time to negotiate a final, broad settlement governing the scope of Iran's nuclear programme.
Giving details about the deal, Deputy Foreign Minister Araqchi told state television that each party's commitments would be implemented "in one day".
"After the first step is taken, then in a short period of time we will again start our contacts for resumption of negotiations for the implementation of the final step."
He added: "We don't trust them... Each step has been designed in a way that allows us to stop carrying out our commitments if we see the other party is not fulfilling its commitments."
Under the terms of the interim deal, Iran must limit its enrichment of uranium to 20 percent fissile purity for a period of six months as the price for relaxation of some sanctions.
Ali Akbar Salehi, head of Iran's atomic energy organisation, said on Saturday it would have no choice but to step up enrichment to the 60 percent threshold if a bill now moving through parliament is approved - even though it has no current need for such highly-enriched uranium.
The bill's supporters say uranium refined to 60 percent concentration would be used to fuel nuclear-powered submarines. That would put Iran on the technical verge of 90 percent fissile purity, which is enough for the core of an atom bomb.
The measure has received expressions of support from at least 218 of parliament's 290 members and, if passed, could threaten progress toward a resolution of the nuclear dispute.
The parliament is much more hawkish than moderate President Hassan Rouhani on the nuclear issue.
But some see the proposal, put forward last month, as a response to a bill introduced by hardline conservatives in the U.S. Senate that would impose stiffer sanctions on Iran, which Western diplomats fear would shatter the nuclear diplomacy.