A complex deal on Iran's nuclear programme can be reached only if global powers stop pressuring Tehran, Foreign Minister Mohammad Javad Zarif warned Saturday evening.

"If the Western countries want to negotiate with the Islamic Republic of Iran, they must make a political decision, which for some could be difficult, and stop with the pressure," Zarif told Iranian state television.

His comment came as Iran's deputy foreign minister Abbas Araghchi prepared for talks in Geneva Sunday with the so-called P5+1 group of global powers towards a comprehensive deal which would rein in Tehran's nuclear programme in exchange for relief from a tight network of sanctions.

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.

Araghchi, who has spent three days meeting with senior US officials and has also met with Russian officials in preparation for Sunday's talks, meanwhile told the Fars news agency a deal would depend on Washington showing "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," he said, while acknowledging that "problems, chasms and differences also exist."

A Western source close to the talks however said the talks 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 would be at least one year to give the international community enough time 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 adamantly denies it is pursuing atomic weapons, insisting it is interested only in nuclear energy.

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

The new Republican-controlled US Congress is considering a fresh sanctions bill, despite strong opposition from President Barack Obama, who has threatened to veto any such legislation that lands on his desk.

If a sanctions bill does go through, some Iranian lawmakers hinted this week that they could push to resume unlimited uranium enrichment.

A flurry of diplomatic activity in the lead-up to Sunday's talks has sought to break the stalemate.

US Secretary of State John Kerry met with Zarif for a marathon six-hour round in Geneva on Wednesday, and again in Paris on Friday.

Zarif also went to Berlin this week to meet his German counterpart Frank-Walter Steinmeier, who stressed that the negotiations had entered a "decisive phase" and urged all parties to "leave nothing undone to reach the solution that has eluded us in recent years."

Iran's top diplomat also met in Paris with his French counterpart Laurent Fabius, who raised "the significant questions that remain to be solved", the French foreign ministry said.