WASHINGTON  - Iran will begin to dismantle some of its nuclear programme beginning Jan. 20 under the terms of an agreement reached with world leaders, President Barack Obama said Sunday, pledging to veto any Congressional efforts to impose new sanctions during talks for a broader agreement.

The implementation plan marks the first time in a decade that Iran “has agreed to specific actions that halt progress on its nuclear program and roll back key parts of the programme,” Obama said in a statement Sunday that confirmed a report made by Iranian officials on Jan 10.

 Iran and the so-called P5+1 countries - China, France, Russia, the UK and the US, plus Germany - reached an accord on Nov 24 in Geneva that hadn’t taken effect. Under the first-step agreement, Iran will get as much as $7 billion in relief from economic sanctions over six months, the Obama administration has said. As part of the agreement, Iran agreed to limit its enrichment capabilities and will undergo more frequent inspections of its nuclear sites, Obama said Sunday.

“Taken together, these and other steps will advance our goal of preventing Iran from obtaining a nuclear weapon,” Obama said. The sanction relief will occur “so long as Iran fulfils its obligations and as we pursue comprehensive solutions to Iran’s nuclear programme.” Obama said he would veto any legislation to impose new sanctions during the negotiations for a broader agreement.

“We are clear-eyed about the even greater challenges we all face in negotiating a comprehensive agreement,” Secretary of State John Kerry said in a statement. “These negotiations will be very difficult, but they represent the best chance we have to resolve this critical national security issue peacefully, and durably.”  As part of the arrangement, Iran will get access to $4.2 billion in oil revenue held in foreign banks. That will be released in regular installments throughout the six months, Kerry said. Iran’s oil exports, the country’s largest foreign-currency earner, plunged last year as US and European Union sanctions meant that banks and insurers couldn’t handle Iranian oil sales.

On the other hand, Iran and the United States said Sunday that a landmark deal Tehran struck with world powers on its disputed nuclear programme will take effect from a January 20 target date.

Under the deal reached in November, Tehran agreed to curb parts of its nuclear drive for six months in exchange for receiving modest relief from international sanctions and a promise by the so-called P5+1 powers not to impose new measures against its hard-hit economy. “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 but President Barack Obama warned there was still a rough road ahead before a comprehensive solution can be nailed. “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 said in the same statement: “With today’s agreement, we have made concrete progress. I welcome this important step forward, and we will now focus 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.” After two days of exhaustive talks, Iran and the European Union agreed Friday on how to implement the deal on containing Tehran’s nuclear programme. The EU represents the P5+1 — Britain, China, France, Germany, Russia and the United States — in the decade-long nuclear negotiations.

On Friday Araqchi had said solutions were found for unspecified points of disagreement but that each country must approve the deal for it to take effect. On Sunday IRNA quoted him as saying: “Finally today we reached an agreement with P5+1 on how to implement the first phase of the agreement.” “The agreement and solutions we found in Geneva were accepted by six countries and on this side, the relevant bodies made the necessary evaluation and also gave them their approval,” the website of state television quoted him as saying.

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 target date of January 20 was reached at the last round in Geneva. 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.

Western nations and Israel have long suspected Iran of pursuing a nuclear weapons capability alongside its civilian programme, charges Tehran strenuously denies. Iran’s nuclear chief Ali Akbar Salehi defended Tehran’s “right” to carry out reasearch on advanced centrifuges. “Advanced centrifuges, which are Iran’s right (to use), were one of the points of disagreement raised by the other party,” Salehi was quoted as saying by Fars news agency.