The White House urged Congress on Thursday not to tie its hands in nuclear talks with Iran, as hawks sought stronger sanctions and to limit its power to offer inducements to Tehran.

As negotiators from Iran and major world powers, including the United States, edged close to an interim nuclear deal in Geneva, skepticism remains rife in Congress.

A group of bipartisan senators is calling for tougher sanctions against Tehran, even as talks take place in Geneva.

And Republican Senator Bob Corker raised the prospect of legislation to limit Obama's power to waive some existing sanctions, as part of an interim deal designed to test Iran's willingness to comply with restrictions on its nuclear program.

"It is important for Congress to reserve its ability to legislate for the moment when it's most effective in order to give the current P5 plus one negotiations the best chance to make real progress," White House spokesman Jay Carney said.

"Our view has been that we need to allow for a pause here so that we can explore the potential represented by these serious and substantive negotiations," Carney said, when asked about the threat of tougher sanctions than those already hurting Iran's economy.

Both Obama and Iranian President Hassan Rouhani find themselves challenged by hardliners in their respective capitals as they pursue a deal to end a long nuclear showdown short of US military action.

Congressional sources say there remains little support for any easing of sanctions at this point, but substantial backing for toughening the regime among lawmakers who think it will make Iran more compliant to Western demands.

But the White House fears Iran will interpret a new layer of sanctions as a sign it is not serious in the talks. Officials also fear tough action by lawmakers will bolster conservatives in Tehran and undercut Rouhani's diplomatic initiative.

Carney stressed that Obama had no intention at this point of softening the "core architecture" of sanctions with Tehran.

But he said Washington was contemplating "limited, targeted and reversible relief" in return for Iran taking steps to halt advances in its nuclear program which the West believes is geared towards producing weapons.

Carney and other US officials declined to offer details of the proposed deal.

Reports from Geneva suggest those steps will include limits on Iran's enrichment capacity, verification steps and restrictions on its stockpiles of nuclear material and its nuclear facilities.

The interim agreement would be designed to build confidence on both sides with a view to concluding a final deal.

Carney said the president envisaged only limited relief for Iran from economic sanctions, so pressure could be reapplied if necessary.

"If Iran does not live up to its commitments, the temporary modest relief would be terminated, and we would be in a position to ratchet up the pressure even further by adding new sanctions."

But key figures, from both parties on Capitol Hill, want to ratchet up pressure on Iran regardless.

Bob Corker, the top Republican on the Senate Foreign Relations Committee, is contemplating a measure that could limit Obama's flexibility in a limited effort to ease sanctions.

"We've crafted an amendment to freeze the administration in and make it so they are unable to reduce the sanctions unless certain things occur," Corker told the Daily Beast website.

"They have the ability now to waive sanctions. But we're very concerned that in their desire to make any deal that they may in fact do something that is very bad for our country."

One option for Corker would be to attach a measure to a bill setting the budget for the Pentagon, which would make it tough for Obama to veto without throwing financing for the entire US military into chaos.

Carney would not say whether Obama would veto such a measure if it reached his desk. But he hinted at something White House officials say privately -- that thwarting the negotiating process could put Washington and Tehran on a dangerous path.

"The willingness to pause while we explore this possibility is really a demonstration of support for the possibility of a peaceful negotiated resolution to this issue rather than a march to war," he said.

International sanctions against Iran
Geneva (AFP) Nov 07, 2013 - International sanctions against Iran focus on key sectors such as defence, finance and oil with the aim of persuading Tehran to end its controversial nuclear programme.

The United Nations, the United States and the European Union have all adopted increasingly tough measures over the past seven years.

On October 3, US President Barack Obama's administration urged lawmakers not to impose more sanctions on Iran as it seeks to respond to overtures from its new President Hassan Rouhani.

-- UNITED NATIONS --

Since 2006, the UN Security Council has approved four series of sanctions:

- Resolution 1737 (December 23, 2006) imposes economic and commercial sanctions against 10 entities linked to Tehran's nuclear and ballistic programmes. The assets of these entities and of 12 prominent figures are frozen.

- Resolution 1747 (March 24, 2007) freezes the assets of 13 new entities linked to the nuclear programme or the Iranian Revolutionary Guards. There is also an embargo on Iranian arms purchases and restrictions on loans to Iran.

- Resolution 1803 (March 3, 2008) hits more entities and individuals with a foreign assets freeze and travel ban. It also bans the supply of dual use items (civil and military) to Iran.

- Resolution 1929 (June 9, 2010) places new restrictions on Iranian investments and bans the sale to Iran of certain heavy arms (tanks, combat planes and helicopters). More names added to sanctions list.

-- UNITED STATES --

Washington first imposed sanctions in the 1980s, banning businesses and individual Americans from trading with Iran except with Treasury Department approval.

- In 2008, the United States introduces further financial restrictions by banning American banks from acting as an intermediary for the transit of funds to and from Iran

- In July 2010, a law targets the supply of petrol to Iran, which is highly dependent on refined products, and punishes foreign groups investing in the Iranian oil sector.

- In November 2011, Washington beefs up its sanctions against individuals supporting the development of Iran's oil sector. In December 2011, the assets of foreign financial institutions that trade with the Iranian Central Bank in the petrol sector are frozen.

- July 31, 2012: Obama imposes new economic sanctions on Iran's oil export sector and on a pair of Chinese and Iraqi banks accused of doing business with Tehran.

- June 3, 2013: Washington announces new sanctions focused on the rial currency for the first time and also the auto sector.

- July 31, 2013: Three days before the investiture of new president Rouhani, the US House of Representatives approves new sanctions which would slap strict limits on Iran's oil industry, the automobile and mining sectors. The legislation has yet to pass the Senate.

-- EUROPEAN UNION -- =

- July 26, 2010, the EU goes further than the UN, banning technical assistance or the transfer of oil technologies to Iran and the activity of some Iranian banks and adds names to the United Nations list of individuals banned from travelling. The measures focus on the Iranian Revolutionary Guards.

- In May and December 2011, the assets of 243 Iranian entities and around 40 more individuals are frozen and visa bans imposed.

- January 23, 2012. The EU agrees on a ban on Iran oil imports, effective July 1, and freezes the assets of the Iranian Central Bank.

- October 15, 2012: A new package of sanctions targets EU dealings with Iran's banks, shipping and gas imports. A government minister is on the list.

- December 21, 2012: The EU adds 18 companies or institutions and one person to a blacklist of those targeted by an asset freeze and travel ban.