Before leaving London for nuclear talks with Iran in Geneva, US Secretary of State John Kerry warned that time was short in the negotiation process, and that much ground still needed to be covered.

"There are still significant gaps, there is still a distance to travel," Kerry told a press conference at the US embassy in London late on Saturday.

The talks, already extended once, are approaching a March 31 deadline for the framework of a deal between Iran and the international community. A further four months, until the end of June, are envisioned for ironing out the details if a framework agreement can be reached.

"President [Barack] Obama has no inclination whatsoever to extend these talks beyond the period that has been set out," Kerry warned.

New faces, even at long-running talks

US and Iranian negotiators began the current round of talks on Friday at the UN's International Atomic Energy Agency (IAEA) headquarters in Switzerland. Senior negotiators from the P5+1 - permanent UN Security Council members China, France, Russia, the UK and the US, plus Germany - will arrive in Geneva on Sunday.

Kerry said there was "absolutely no divergence whatsoever in what we believe is necessary for Iran to prove that its nuclear program is going to be peaceful," concluding that "the P5+1 remains united on the subject of Iran." On other matters, most notably Ukraine, this unity has been lacking of late.

On the Iranian side, nuclear energy chief Ali Akbar Salehi and Foreign Minister Mohammad Javad Zarif will attend, along with President Hassan Rouhani's brother and close aide, Hossein Fereydoun.

Salehi is joining the talks in Geneva for the first time, as is his US counterpart, Energy Secretary Ernest Moniz, prompting some observers to suggest possible progress; but Kerry urged caution.

"I would not read into it any indication whatsoever," Kerry said, saying the "technical" nature of the negotiations could also explain their presence. Stumbling blocks in the talks include the number of centrifuges Iran can operate to enrich uranium, how much uranium it can stockpile and what research and development it can pursue in the field of uranium enrichment.

The current diplomatic efforts had been billed as a key stage in a 12-year standoff over Iran's nuclear program, which Tehran says is for power-producing and medical purposes only. The talks have been tied to the possibility of the US rolling back sanctions against Iran, although this promise has come under fire in the US Congress.

Opposition Republicans have invited Israeli Prime Minister Benjamin Netanyahu to address Congress in Washington early next month, in the run up to elections in Israel; he is expected to criticize continuing negotiations with Tehran.

msh/sms (AFP, AP, Reuters)