WASHINGTON/Tehran  - The United States set out what it called its ‘bottom lines’ to reach a deal with Iran to rein in its nuclear programme, ahead of new talks next week.

Washington had stuck to its guns that it wanted a ‘good deal’ and had agreed to several extensions of the negotiations ‘because we have held firm to certain bottom lines,’ a senior US administration official said. ‘We will only accept an agreement that cuts off the different pathways to the fissile material that Iran needs for a nuclear weapon,’ the official stressed.

US Secretary of State John Kerry will leave at the weekend for Switzerland, where he will meet once again with his Iranian counterpart Mohammad Javad Zarif. The pace and intensity of the talks have sped up as global powers gathered under the P5+1 group have been seeking a deal with Iran to stop it acquiring a nuclear weapon - something Tehran denies trying to do.

A March 31 deadline for a political framework for the deal is looming with negotiators saying they will aim to pin down the final technical details by June 30. While US officials cautioned that there were no guarantees a deal would be reached, they said ‘the negotiations have advanced substantially, gaps have narrowed.’ The officials were talking only days ahead of a controversial visit to Washington by Israeli Prime Minister Benjamin Netanyahu, who has vehemently opposed a nuclear deal with Iran.

Netanyahu is due to address the US Congress on Tuesday on why he believes the Iran deal is a bad move, despite opposition from the White House which sees it as interference ahead of Israel's elections later in March. ‘When we have an agreement it has to be measured against the alternatives,’ the US administration official said. The lack of an agreement could present the US with the question of whether or not to use ‘military force’ to halt a resumed Iranian nuclear program. Staking out the broad outlines of a deal - without going into specifics - the US official spelled out where Washington was standing firm: Iran should not be allowed to develop weapons-grade plutonium at its Arak reactor. ‘We're discussing how Iran can convert that Arak reactor to serve a different purpose,’ the official said. - Iran should not use its Fordo nuclear plant to enrich uranium.

As US and Iranian officials inch towards an agreement on Iran’s nuclear enrichment programme, a group of American senators has introduced a bill that would allow a congressional review of any deal which is concluded with Tehran.

The legislation would require President Barack Obama to submit the text of any deal to Congress and would ban the White House from lifting any sanctions for a period of 60 days so that Congress could hold hearings and debate the deal. The move dubbed the ‘Iran Nuclear Agreement Review Act of 2015’ was introduced on Friday by Senators Bob Corker, Robert Menendez, Lindsey Graham and Tim Kaine.

Sens. John McCain, Joe Donnelly, Marco Rubio, Heidi Heitkamp, Kelly Ayotte, Bill Nelson, Jim Risch, and Angus King co-sponsored the bill.

‘There are few national security priorities for our country more important than’ Iran deal, Corker said, adding any agreement ‘must include Congress having a say on the front end.’

Menendez said, ‘As we enter the final weeks of negotiations, Congress is rightly pursuing a dual track approach to the Iran nuclear issue and applying responsible pressure on Iran to ensure the right outcome is reached at these talks.’ Senator Graham described the stakes of the ongoing negotiations with Iran as important to the US national security in such way that Congress should review and vote on any deal before it becomes binding.

Congressional aid did not know when the bill would come to a vote in the Senate Foreign Relations Committee. The legislation is seen as a challenge to Obama’s authority over a possible deal with Tehran. Nuclear talks between representatives from Iran and the United States started on February 22 in the Swiss city of Geneva and the two sides held three rounds of talks in two days in an attempt to narrow differences ahead of a key July 1 deadline for reaching a comprehensive deal.Both the Iranian and US top diplomats said some progress was made toward a final agreement.

Moreover, Iran on Saturday shrugged off a bid by Israeli Prime Minister Benjamin Netanyahu to abort a nuclear deal between Tehran and world powers by lobbying opposition in a speech to the US Congress.

‘I believe this effort is fruitless and it should not be an impediment to an agreement,’ Iranian Foreign Minister Mohammad Javad Zarif said at a joint press conference with his visiting Italian counterpart Paolo Gentiloni. ‘It is unfortunate that there is a group which sees its interests in tension and crisis.’ Netanyahu's bid was ‘an attempt to utilise a fabricated crisis to cover up realities in the region, including occupation, the suppression of Palestinians and the violation of their rights’, he said.

‘It is an on old policy to intimidate and spread lies in order to prevent peace in the region,’ said Zarif. Netanyahu will travel next week to Washington to denounce a possible agreement in the Iranian nuclear talks, which he considers contrary to the interests of Israel. The Israeli leader said Wednesday that his speech before Congress was part of his ‘duty’ to protect the Jewish state's security.

‘Under the agreement that is being prepared, we have reason to worry if the world powers have apparently found common ground with Iran,’ he said. The so-called P5+1 group of Britain, China, France, Russia, the United States and Germany are trying to strike an accord that would prevent Tehran from developing a nuclear bomb. The next round of talks is to start next week in Switzerland.

In return, the West would ease punishing sanctions imposed on Tehran over its nuclear programme, which Iran insists is purely civilian. A March 31 deadline for a political framework for the deal is looming with negotiators saying they will aim to pin down the final technical details by June 30. Zarif said that Iranian and US delegations meeting in Geneva last week had made ‘significant progress’ on technical issues.

‘But still differences remain on many other topics, both on nuclear issues and on sanctions. The United States and the West must realise that sanctions are an obstacle to reaching an agreement,’ he said. Italy's foreign minister, for his part, called for ‘a solution on the nuclear issue’ to allow for ‘expanded relations’ between Tehran and Rome, according to a Persian translation of his remarks. ‘Iran's help will be effective in finding solutions to regional problems, including the fight against the extremism of Daesh,’ said Gentiloni, using the Arabic acronym for the Islamic Jihad jihadist group.