The extension of international nuclear negotiations with Iran is likely to prompt a far-reaching showdown between the White House and US Republicans who seek to tighten economic screws against Tehran.

With hawkish lawmakers eager to pass new sanctions through Congress to prod the Islamic republic into a lasting deal that prevents Iran from developing a nuclear weapon, experts warned that attempts to impose economic penalties could rip apart the delicate negotiation process.

Iran and world powers agreed to extend terms of an interim deal -- which include limited sanctions relief -- for another seven months as they work toward a final accord, and President Barack Obama has largely been given a free hand by Congress to deal with Tehran over the past year.

With the deadline now postponed a second time, some lawmakers want to hold Iranian leaders' feet to the fire, insisting that since punitive sanctions brought Tehran to the negotiating table, passing new sanctions now will be the key to securing a lasting agreement.

Such a move could "undermine our entire position," George Perkovich, vice president for studies at the Carnegie Endowment for International Peace, told AFP about US negotiating efforts.

"I can guarantee you if Congress passes new unilateral sanctions, the Iranians aren't going to become more accommodating. It'll be quite the opposite."

Not only will Tehran show fury, it could erode international support for the economic embargo and "alienate Turkey, India, China and other countries that you need to uphold sanctions."

Perkovich said Iran's leadership firmly believes Washington wants nothing less than regime change in Tehran, and that congressional intervention in the negotiating process would show them that Congress "can't be relied upon to remove sanctions" in the future.

But Republicans, and some prominent Democrats, are keen to press ahead.

"We believe this latest extension of talks should be coupled with increased sanctions and a requirement that any final deal between Iran and the United States be sent to Congress for approval," Senate Republicans John McCain, Lindsey Graham and Kelly Ayotte said Monday in a statement.

Last year the pro-Israel lobby group AIPAC opposed new sanctions, backing Obama's negotiation strategy. Now the group with sway on Capitol Hill says fresh economic punishment against Iran will "strengthen America's bargaining position."

"It is now essential that Congress take up new bipartisan sanctions legislation to let Tehran know that it will face much more severe pressure if it does not clearly abandon its nuclear weapons program," AIPAC said.

- 'Embarrassing' foreign policy conduct -

Democratic Senator Robert Menendez, the Senate Foreign Relations Committee chairman who with Republican Mark Kirk penned sanctions legislation that had backing from 60 senators but was shelved due to White House pressure, called the failure to secure a deal "disappointing and worrying."

He signaled he could push for re-introduction of his measure when Republicans take Senate control in January.

Incoming Senate Majority Leader Mitch McConnell has not said openly whether he would introduce Iran sanctions legislation, but in February he called the Menendez-Kirk measure "perfectly reasonable."

Obama would likely veto any sanctions during the negotiating period. And while Republicans in the new Congress may yet be able to siphon off enough Democrats to override a veto, "that's an embarrassing way to conduct foreign policy," Perkovich said.

The impact -- even if a veto stands -- could be severe, said John Bradshaw, who heads the National Security Network, a progressive think tank.

"It's not a situation that would inspire a lot of confidence in the Iranians that it's a long-term commitment by the United States, and that they should fulfil their end of a commitment in terms of inspections and suspending enrichment activities," Bradshaw told AFP.

"In the near term, the president has the advantage, but Congress can do a lot to undermine diplomacy."

Bradshaw warned of other "counter-productive" efforts being considered, including a bid to cut off funding for the State Department's work with the International Atomic Energy Agency.