Iran will begin eliminating its stockpile of highly enriched uranium in eight days' time, after negotiators in Geneva reached an agreement on how to implement the interim nuclear deal reached in November.

The six-month deal, under which Iran has agreed to freeze and even curb some nuclear activities in return for an easing of sanctions, will come into force on 20 January.

"As of that day, for the first time in almost a decade, Iran's nuclear programme will not be able to advance, and parts of it will be rolled back, while we start negotiating a comprehensive agreement to address the international community's concerns about Iran's programme," the US secretary of state, John Kerry, said in a statement.

The start-date for the agreement was announced on Sunday by the European Union's foreign policy chief, Catherine Ashton, who represents the US, China and Russia and France, Germany and Britain, the three European powers who reached the settlement with Iran.

President Barack Obama described the latest agreement as a significant step forward, but immediately renewed his plea for Congress not to introduce a range of new sanctions against Iran, which he said would risk "derailing our efforts to resolve this issue peacefully".

Sunday's announcement, which was confirmed by Iran, marked the culmination of weeks of fine-tuning over the technical implementation of November's interim deal. Senior officials from the EU and Iran met in Geneva on Thursday and Friday, making progress on a framework to execute the nuclear deal that was ratified by participating countries over the weekend.

The breakthrough comes at a critical moment for the nuclear agreement, which is coming under growing pressure from hardliners in both Tehran and Washington.

November's agreement provides limited sanctions relief to Iran – which, according to US estimates, will provide a $7bn boost to the country's economy. In return, Iran has agreed to freeze its nuclear programme, destroy stockpiles of higher-grade uranium and commit to more rigorous inspections.

The Obama administration will hope the details of the implementation of the deal will reassure sceptics in Congress. A senior administration official said Iran had agreed to disable the cascade that produces 20%-enriched uranium before the start of the agreement, and to begin destroying existing stockpiles immediately.

"By the end of the six months, it will all be gone," a second senior administration official said.

In return, world powers will also suspend some petrochemical and auto sanctions on 20 January. However, the bulk of relief will be in the shape of $4.2bn in restricted Iranian assets that will be repatriated to Tehran in regular instalments throughout the six months until the deal concludes in July.

Some of those funds are being set aside to be released to Iran as and when it completes its promised destruction of higher-grade uranium.

"We are basically waiting to make certain that Iran has begun to fulfil its commitments, as verified by the IAEA [International Atomic Energy Agency], before the funds are made available to the Iranians," a third senior administration official said.

All three officials spoke to reporters on a conference call on the condition of anonymity – a routine requirement the Obama administration places on such calls.

They said most of the fine print hammered out in recent days relates to the IAEA, which will begin an expanded series of inspections of Iran's nuclear facilities, providing monthly updates to a joint commission overseeing the agreement.

The agreement will last only until July, and officials close to the negotiations say it will almost certainly have to be renewed to give more time for extended talks which, the parties hope, will yield a permanent settlement to end the standoff over Iran's nuclear capabilities.

But the deal has received a frosty reception in Washington, where senior Republicans and Democrats are threatening to introduce a bill to ramp up sanctions against Tehran – a move experts say would breach the spirit if not the letter of November's agreement.

"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 on Sunday.

He added: "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."

The White House has so far dissuaded senior lawmakers from pursuing a bill to tighten the sanctions noose on Iran. Despite November's breakthrough, there had been doubts over whether senior diplomats would succeed in agreeing a plan to execute the historic agreement.

Ayatollah Ali Khamenei, Iran's supreme leader. Photograph: AFP/Getty Images

The enthusiasm that greeted the deal in November has been short-lived and on Thursday Iran's supreme leader, Ayatollah Ali Khamenei, used fiery language to criticise the approach of the US – which he called the "Great Satan" – to the negotiations.

"The nuclear talks showed the enmity of America against Iran, Iranians, Islam and Muslims," he said.

Khamenei's decision to give Iran's reformist president, Hassan Rouhani, space to reach a deal with the US and other powers was crucial to November's deal.

When Kerry tried to sell the plan to Congress last month he met a wall of opposition from both parties, who appeared almost entirely unified against the deal, which some argued had endangered the US and its ally Israel.

In his statement on Sunday, Kerry echoed Obama's call on Congress to hold back on new sanctions. "Now is not the time for politics," he said.

Any bill increasing sanctions on Iran would likely contain a six-month delay, and only be implemented if the current agreement fell apart. However, it would jeopardise November's deal, which contains a clause in which the US administration committed itself to "refrain from imposing new nuclear-related sanctions" – even though the wording provided some acknowledgement that Obama could not control legislation passed by Congress.

In response, a group of Iranian MPs has threatened to propose a bill requiring the Rouhani government to enrich uranium up to 60%, an unacceptable level for the west, should the US decide to impose new sanctions.