US lawmakers are aiming to tighten sanctions on Iran to prevent Washington giving away too much in a deal on Tehran's nuclear programme that diplomats said was still possible despite the failure of high-level weekend talks.

The lawmakers' moves reflected widespread Congressional scepticism about a rapprochement between Iran and world powers and coincided with renewed lobbying from Israel against a proposal it sees as leaving open a danger Iran could build a nuclear bomb. Tehran denies harbouring any such ambition.

Negotiators from world powers will resume talks with Iran in 10 days after failing late on Saturday to reach agreement on an initial proposal to ease international sanctions against Tehran in return for some restraints on its nuclear programme.

The new talks will be at a lower level than the foreign ministers who gathered in Geneva at the weekend, but Britain and Russia both said the chances for a deal were fairly high.

The sides seemed on the verge of a breakthrough - before cracks materialised among US and European allies as French Foreign Minister Laurent Fabius dismissed the plans as a "fool's game" of one-sided concessions.

The Senate Foreign Relations Committee will move ahead with additional sanctions this week to keep the pressure on Iran as talks continue, said Democratic Senator Bob Menendez, the committee's Democratic chairman.

"My concern here is that we seem to want the deal almost more than the Iranians," Menendez said on ABC's "This Week."

US Secretary of State John Kerry, who joined the Geneva negotiations unexpectedly on Friday to help bridge differences, defended the administration's position.

"We are not blind, and I don't think we're stupid," he said on US television. "I think we have a pretty strong sense of how to measure whether or not we are acting in the interests of our country and of the globe."

IRANIAN "RED LINES"

For his part, Iranian President Hassan Rouhani sought to win hardliners in parliament over to his diplomatic opening to world powers, said it had "red lines".

"We will not answer to any threat, sanction, humiliation or discrimination," was the message Iranian negotiators had told their big power interlocutors in Geneva, Rouhani said.

Israeli Prime Minister Benjamin Netanyahu, whose country is widely believed to be the Middle East's only nuclear-armed power, reiterated a veiled threat to take military action if it deems diplomacy to restrain Iran's nuclear ambitions a dead end.

Rouhani may also have been alluding to pressure from US and Israeli hawks for Iran to scrap its whole nuclear programme.

Diplomats said France wanted any deal to require a shutdown of Iran's Arak heavy-water reactor - of potential use in making bomb-grade plutonium - and the removal of Iran's stockpile of higher-enriched uranium. Another stubborn issue was the extent and sequencing of relief from sanctions demanded by Tehran.

Netanyahu said on Sunday it was good that no deal with Iran was clinched at the weekend and that he had lobbied against scaling back sanctions by telling leaders: "What's the rush?"

The right-wing Israeli premier took his case straight to the American public, appearing on network television to decry "a very bad deal" he feared was in the making.

That any deal might be feasible after a decade of increasingly heated confrontation between Iran and Western powers, shows the striking shift in the tone of Iranian foreign policy since Rouhani's landslide election victory in June.

Rouhani, a relative moderate, opened diplomatic windows to a nuclear deal in order to alleviate sanctions that have throttled OPEC giant Iran's lifeblood oil industry and cut it off from the international banking system.

He has won crucial public backing from Supreme Leader Ayatollah Ali Khamenei, Iran's ultimate authority, who - despite his profound suspicion of Washington - has warned hardline loyalists not to discredit the negotiating path.

TEMPORARY DEAL

Rouhani has repeated Iran's longtime insistence on a right to sovereign nuclear energy for peaceful purposes as a member of the Non-Proliferation Treaty.

Most diplomats acknowledge that, since Tehran has expanded its nuclear capacity exponentially since 2006 and publicly equated the programme with national pride and progress, the time for demanding its total shutdown - although enshrined in several U.N. Security Council resolutions - has now passed.

The powers remain concerned that Iran is continuing to amass enriched uranium not for future nuclear power stations, as Tehran says, but as potential fuel for nuclear warheads.

They are searching for a preliminary agreement that would cap Iran's nuclear capacity and open up the programme to UN anti-proliferation inspectors. In exchange, they have offered phased, limited and reversible relief from sanctions. Iran, however, wants an early end to oil and banking sanctions.

Under discussion is a temporary deal entailing a freeze to higher-grade uranium enrichment - which Iran bills as fuel for a medical research reactor but which is also potential material for bombs - lasting about six months.

During that time, Iran and the six powers would negotiate a permanent agreement aimed at ensuring that none of Iran's nuclear activities could be diverted towards bomb-making.

Given the progress in Geneva, UN nuclear watchdog chief Yukiya Amano said he hoped a meeting he would have in Tehran on Monday would yield results in his effort to overcome Iranian resistance to an investigation into its nuclear work. - Reuters