The nuclear talks between Iran and six world powers have been extended beyond Tuesday midnight deadline in an effort to hammer out the outline of an agreement on curbing Tehran's nuclear programme.

US State Department spokeswoman Marie Harf said on Tuesday that the extension was justified although "several difficult issues" still needed to be bridged.

The decision came after six days of marathon efforts to reach a preliminary understanding by Tuesday midnight, drawing in foreign ministers from Britain, China, France, Russia, the US and Germany - collectively known as P5+1 - and Iran.

Iranian Foreign Minister has said that he hopes to finalize on Wednesday a framework nuclear agreement with world powers.

"We have accomplished quite a bit but people needed to get some rest and start over early in the morning," Mohammad Javad Zarif told reporters in the Swiss city of Lausanne on Wednesday.

"I hope that we can finalize the work on Wednesday and hopefully start the process of drafting (a final accord)" by the target date of June 30, Zarif said as the marathon talks ended for the night.

His comments were echoed by Russian Foreign Minister Sergei Lavrov who said that global powers had reached an agreement in principle on "all key aspects" of the outlines of the nuclear deal.

"One can say with relative certainty that we at the minister level have reached an agreement in principle on all key aspects of the final settlement of this issue, which will be put on paper in the coming hours or perhaps within one day," Russian news agency, Ria Novosti, quoted Lavrov as saying.

The two sides hope to reach a preliminary understanding that will allow them to enter a new phase of negotiations aiming at a final deal by June 30.

US Secretary of State John Kerry, who had planned to leave the talks on Tuesday, stayed back.

The talks are aimed at stopping Iran from gaining the capacity to develop a nuclear bomb in exchange for the easing of international sanctions crippling its economy.

Obstacles remain on several main issues - uranium enrichment, where stockpiles of enriched uranium should be stored, limits on Iran's nuclear research and development and the timing and scope of sanctions among other issues, according to negotiators.

For weeks, US Secretary of State John Kerry and his Iranian counterpart, Mohammad Javad Zarif, have been meeting in an intense effort to reach a political understanding on terms that would curb Iran's nuclear activities in exchange for sanctions relief.

"We're working hard, very hard," Kerry told Al Jazeera on Monday.

Sticking points

Tehran has said it is willing to address concerns about its stockpiles of enriched uranium, although it has denied that will involve shipping it out of the country, as some western officials have said.

A senior State Department official said that shipping the stockpile is one of the "viable options that have been under discussion for months ... but resolution is still being discussed".

Meanwhile, Israel, which is at loggerheads with Iran, said the deal being negotiated was worse than it feared.

"I expressed our deep concern towards this deal emerging with Iran in the nuclear talks, this deal as it appears to be emerging bears out all of our fears, and even more than that," said Prime Minister Benjamin Netanyahu.

Israel and the US insist Tehran is seeking to build a nuclear bomb, but Tehran says its nuclear programme is intended for power generation.

PHOTO CAPTION

Iranian Foreign Minister Mohammad Javad Zarif (L) greets Atomic Energy Organization of Iran head Ali Akbar Salehi at the Beau Rivage Palace Hotel on March 31, 2015 in Lausanne.

Aljazeera