Direct talks with the United States this week on Tehran's nuclear programme hold the key to bridging gaps at a "serious phase" of negotiations and sealing a deal, a top Iranian official said Sunday.

The two countries will hold their first full-scale official direct meetings in decades on Monday and Tuesday in Geneva, with the route toward an eventual lifting of sanctions expected to be the main issue.

Abbas Araqchi, a vice foreign minister who will lead the Iranian delegation, said the tete-a-tete with the United States was essential, as the negotiations are delicately poised.

The P5+1 group of permanent members of the Security Council -- Britain, China, France, Russia and the United States -- plus Germany have long sought to reach a settlement over Iran's nuclear programme.

But with the last round of talks in Vienna in May yielding next to no progress, there has been concern that the P5+1 process was stalling.

The announcement on Saturday of the US-Iran meetings in Geneva came as a surprise, but appeared to confirm the need for secondary steps to close big gaps between Tehran and Washington's positions.

"We have always had bilateral discussions with the United States in the margin of the P5+1 group, but since the talks have entered a serious phase, we want to have separate consultations," Araqchi said, quoted by official IRNA news agency.

"Most of the sanctions were imposed by the US, and other countries from the P5+1 group were not involved," he added, in a telling remark about how the US stance remains Iran's main concern.

- US team known -

The US team in Geneva will be led by Deputy Secretary of State Bill Burns and Jake Sullivan, a top White House adviser.

The two Americans were part of a small team who through months of secret talks in Oman managed to bring Iran back to the P5+1 negotiating table last year.

Araqchi welcomed Burns's presence, saying he hoped it would be "as positive during these negotiations" as previously.

A senior US administration official said Saturday that the Geneva talks would "give us a timely opportunity to exchange views in the context of the next P5+1 round in Vienna," between June 16-20.

The talks are aimed at securing a comprehensive agreement on the Islamic republic's nuclear activities, which the West suspects is aimed at developing weapons, but which Iran insists is for peaceful purposes.

After decades of hostility, Iran and the US made the first tentative steps towards rapprochement after the election of self-declared moderate Hassan Rouhani as president last June.

Rouhani called his US counterpart Barack Obama shortly after he took office, which was followed by a meeting between US Secretary of State John Kerry and Iran's Foreign Minister Mohammad Javad Zarif.

- 'Stubbornly recalcitrant' -

An interim deal struck last November led the US and its partners to release $7 billion from frozen funds in return for a slowdown in Iran's controversial uranium enrichment.

But a long-term accord, ahead of a July 20 deadline, remains a long way off, experts say.

Cyrus Nasseri, a member of Iran's nuclear negotiating team when it was led by Rouhani between 2003 and 2005, told AFP the US role as "the main interlocutor" explained the need for direct talks, and said Washington had to drop its "stubbornly recalcitrant" outlook.

"It's all a matter of whether the US will be prepared to take the next step to accept a reasonable solution which will be win-win for both," with Iran allowed to maintain a uranium enrichment programme, he said.

"The US has to bite the bullet after 10 years of wrongful accusations. It has to accept Iran will at the end of day, no matter how the settlement is made, have peaceful nuclear fuel production."

Mehdi Mohammadi, a member of the nuclear negotiating team that preceded Zarif's, said Araqchi's negotiators were in a good position.

But he predicted that any deal will only be reached "in the 90th minute, when the talks would appear to be close to collapsing", using a football analogy.