World powers and Iran hold a second day of talks Wednesday aimed at ending the decade-long standoff over Tehran's nuclear programme, after the Islamic republic put what it billed as a breakthrough proposal on the table.

Top officials backed by experts on nuclear affairs and sanctions were combing through the details of Iran's proposal which it said allowed for snap inspections of its atomic sites.

Iran's negotiations in Geneva with European Union-chaired P5+1 group -- the United States, Britain, France, China and Russia, plus Germany -- started Tuesday. The timing of Wednesday's sessions remained unclear.

All sides underscore the changed tone in the talks, which come two months after Iranian President Hassan Rouhani, seen as a relative moderate, succeeded conservative Mahmoud Ahmadinejad.

The meeting ended a six-month freeze in negotiations after Iran's refusal to curb uranium enrichment in exchange for easing the international sanctions battereing its economy.

Western powers and Iran's archfoe Israel fear that Tehran's atomic programme is a disguised effort to build a nuclear bomb, a claim it denies vehemently.

Rouhani has pledged transparency on the nuclear programme and engagement with the international community to try to get the sanctions lifted -- though Israel has warned against being taken by his "sweet talk".

"We are very serious. We are not here symbolically, to waste our time," Iran's lead negotiator Abbas Araqchi said Tuesday.

On Tuesday, Iranian Foreign Minister Mohammad Javad Zarif and his team made an hour-long presentation to the P5+1.

"The proposal that we have introduced has the capacity to make a breakthrough," Iran's lead negotiator Abbas Araqchi said Tuesday, telling reporters it was "very comprehensive" but that all parties had agreed to keep it under wraps.

There was confusion about what Iran had on offer, with Iranian state news agency IRNA quoting Araqchi as saying Tuesday that an additional protocol to the nuclear Non-Proliferation Treaty allowing snap inspections was not part of the offer.

On Wednesday he clarified his remarks.

"None of these issues exist in the first step, but they are part of our last step," IRNA quoted him as saying.

Iran has already drawn red lines, however, saying it will not accept demands to suspend uranium enrichment or ship stockpiles of purified material abroad.

Earlier, Zarif said Tehran's plan contained three steps that could settle the long-running nuclear standoff "within a year", with the first achievable "within a month or two, or even less".

Western officials expressed cautious optimism about the potential results of the talks.

US State Department spokeswoman Jen Psaki said that "since the technical conversations and discussions are ongoing, I don't think we'd characterise it as a breakthrough at this stage".

"However, it certainly is positive that there was enough information to have technical discussions," she added.

A senior US official in Geneva said any easing of sanctions would be "proportional" to what Iran pledged, with "concrete, verifiable actions" needed.

Also in Geneva, EU foreign policy spokesman Michael Mann said discussions were "very detailed". He praised the "very different" atmosphere from previous talks but insisted the ball remained in Iran's court.

Israel, the Middle East's sole if undeclared nuclear-armed power, warned Tuesday against accepting "cosmetic concessions" that would not stop Iran from pursuing nuclear weapons. It has not ruled out a military strike to halt Iran's drive.

Given what they say is Tehran's past playing for time via talks, Western negotiators say they are far from naive, and Washington underlines it is serious about never allowing a nuclear-armed Iran.

Iranian negotiators also held a flurry of one-to-one meetings with individual P5+1 delegations in Geneva.

On a key sign of the changed tone, Iran's Araqchi and his US counterpart Wendy Sherman met Tuesday evening in Geneva, in the first direct talks between nuclear negotiators from the two countries since 2009.

Last month, Zarif also held a landmark meeting with US Secretary of State John Kerry amid a P5+1 gathering on the sidelines of the UN General Assembly in New York, while Rouhani spoked on the telephone with US President Barack Obama.