Geneva : US Secretary of State John Kerry and his Iranian counterpart today took a walk by Lake Geneva and then began intense talks here on Tehran's nuclear programme to remove the "significant gaps" that remain ahead of a key deadline.

The bilateral talks between Iranian Foreign Minister Mohammad Javad Zarif Zarif and Kerry have begun while the P5+1 ?- China, Russia, the UK, the US, France and Germany -- were to meet as a group later.

Ali Akbar Salehi, the director of the Iranian Atomic Energy Organisation and Iranian President Hassan Rouhani's brother and close aide Hossein Fereydoon along with US Energy Secretary Ernest Moniz made their maiden appearance in the talks signalling that the deal is entering a sensitive stage.

The two officials and their negotiators spent more than five hours yesterday negotiating technical details of the nuclear talks.

The US and Iranian diplomats along with negotiators from the P5+1 group have been meeting in Geneva since the last three days to smoothen out major bumps in reaching a nuclear deal.

The tricky negotiations have mainly been over Iranian uranium enrichment and the pace of removing sanctions, which the US wants to stagger over time.

The six powerful nations are trying to broker a deal with Iran to end a more than a decade-long standoff over the Islamic Republic's nuclear programme in return for an easing of sanctions.

Iran, however, has maintained that its nuclear programme is for peaceful purposes.

Kerry was to arrive in the Swiss city this morning for two days of talks with Zarif but was delayed in London.

The US Secretary of State had warned yesterday that "significant gaps" remain ahead of the March 31 deadline which the Obama administration has set to agree on a political framework for the deal.

Two deadlines for a permanent agreement have been missed since a November 2013 interim deal in which Iran was given limited sanctions relief in exchange for diluting its stock of fissile material from 20 per cent enriched uranium to five per cent.

Meanwhile, Israeli Premier Benjamin Netanyahu today termed the efforts of world powers to strike a nuclear deal with Iran as "dangerous". He said it was "astonishing" that talks were on even after IAEA found that Tehran was hiding military components of its atomic programme.

A confidential document by the IAEA, distributed among its member states on Thursday, reportedly said that the Islamic Republic was continuing to withhold full cooperation in two areas of a long-running investigation by the UN watchdog that it had committed to presenting by August last year. (PTI)