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 A top Iranian negotiator voiced optimism Saturday that talks with the United States seeking to nail down a complex nuclear deal could succeed if Washington showed "good will".

"We remain hopeful and I think that if the other side has the necessary good will and determination, it will be possible to reach a deal," Iran's deputy foreign minister Abbas Araghchi told the Fars news agency.

Congress needs to show patience. - Barack Obama

His comments came on a third day of talks with senior US officials in Geneva aimed at accelerating progress towards a comprehensive deal which would rein in Iran's nuclear programme in exchange for relief from a tight network of global sanctions.

Iranian negotiators were also set to meet with Russian officials later in the day, before full negotiations on Sunday with the so-called P5+1 group – the United States, Britain, China, France, Russia and Germany.

Following an interim accord in November 2013, two deadlines for a final deal have been missed, and now a third one is looming on July 1.

"The negotiations are continuing in a completely serious atmosphere and both parties want to reach an agreement, but problems, chasms and differences also exist," Araghchi acknowledged.

A Western source close to the talks meanwhile said they did not seem to be moving forward significantly and that the biggest stumbling block was on the Iranian side.

"The Iranians have not yet made enough gestures to enable us to reach a good deal that would ensure a substantial reduction of their residual (uranium) enrichment capacity, so we collectively can be assured they don't have the technical capacity to rapidly develop a nuclear bomb," he told AFP.

Avoiding a 'disastrous' deal

The source argued that any deal needed to ensure that Iran's "breakout capacity" for making a nuclear bomb was at least one year to give the international community time enough to act.

If the global powers go along with "a weak deal, that would send a disastrous signal to the rest of the world on the issue of proliferation," he said.

Iran insists it is only interested in nuclear energy and adamantly denies it is pursuing atomic weapons.

Among issues complicating negotiations are hardliners on both sides who appear willing to torpedo the efforts.

US President Barack Obama on Friday urged Congress not to impose new sanctions on Iran over its disputed nuclear program, threatening to veto any such legislation that lands on his desk.

Obama told a joint press conference with British Prime Minister David Cameron that Iran was already chafing under existing sanctions and had not accelerated its programme, and that he would strongly urge Congress not to torpedo the ongoing talks with Tehran.

"Congress needs to show patience," Obama said.

"We'll see how persuasive I am. But if I'm not persuading Congress, I promise you, I'm going to be taking my case to the American people on this," he warned.

Obama has faced mounting calls from Republican critics for tougher new sanctions on Iran, with lawmakers saying a debate on more stringent measures could take place in the US Senate within weeks.

But new sanctions would "jeopardize the possibility of... providing a diplomatic solution to one of the most difficult and long-lasting national security problems that we've faced in a very long time," Obama said.

"I will veto a bill that comes to my desk."

Cameron also spoke out against calls for further sanctions on Iran, saying negotiations needed "space" to succeed.

"We remain absolutely committed to ensuring that Iran cannot develop a nuclear weapon," Cameron said.

"The best way to achieve that now is to create the space for negotiations to succeed. We should not impose further sanctions now."

Two paths in Congress

However, Republicans in the United States are seeking to shape US policy on Iran by two paths being crafted in Congress.

One tactic envisages adoption of a bill requiring Obama to submit any nuclear accord reached with Iran to Congress for approval.

Another strategy is based on deferred economic sanctions that would be activated if Tehran refuses to sign a final deal or is seen as violating its terms.

A top US senator said debate could begin at the end of January or in early February.

Obama administration officials are vehemently opposed to any new measures.

"I've always said that the chances that we can actually get a diplomatic deal are probably less than 50/50. Iran is a regime that is deeply suspicious of the West," Obama said.

"But if, in fact, we still have an opportunity to get a diplomatic deal that provides us verifiable assurances that they are not developing a nuclear weapon, that is the best possible outcome that we can arrive at right now."

Washington's UN envoy Samantha Power warned Monday that imposing new sanctions "will almost certainly" see to the end of negotiations.