Iran and major powers on Wednesday began the difficult process of finalizing by June 30 a historic deal putting an Iranian nuclear bomb out of reach, three weeks after agreeing the main outlines.

Following a negotiating marathon in Switzerland, Iran agreed on April 2 to what U.S. President Barack Obama called a "historic understanding... which, if fully implemented, will prevent (Iran) from obtaining a nuclear weapon."

Israel's intelligence minister on Monday proposed terms for a final nuclear accord with Iran which he said would be an improvement on the outline drawn up last week.

Yuval Steinitz told journalists that U.S. President Barack Obama's pledge to back Israel's security was appreciated, but it did not outweigh the potential threat of a nuclear-armed Iran.

The leader of Friday weekly prayers in Tehran endorsed as "great" a nuclear deal between Iran and world powers, but demanded that the West keep its promises.

The comments by Ayatollah Mohammad Emami-Kashani, which state media said were replicated nationwide, are a further boost for President Hassan Rouhani's government, which has been pushing for an accord.

As prisons go, the famed Beau-Rivage hotel nestled on the Swiss side of Lake Geneva with breath-taking views over the snow-capped French Alps must count as one of the world's most luxurious.

But after six days and nights of living within the confines of its magnificent 19th century frescos and marbled columns, even top diplomats were beginning to go a bit stir crazy as they seek to hammer out a historic framework deal with Iran.

China is "cautiously optimistic" that global powers can reach a deal with Iran to pare back its nuclear program amid intense negotiations, Chinese Foreign Minister Wang Yi said Monday.

"The key issues are quite clear and the preferred approach of a solution is emerging," Wang told reporters as he left the luxury lakeside hotel in Lausanne, Switzerland, where the talks are being held.

A nuclear deal between Iran and world powers is "doable," Tehran's lead negotiator said in crunch talks in Switzerland on Sunday, with "two or three" issues still to be resolved.

Abbas Araqchi however ruled out sending Iran's nuclear stocks abroad, a key demand of world powers, while insisting that all U.N. Security Council resolutions and sanctions must be lifted.

Prime Minister Benjamin Netanyahu's victory in the Israeli election will not disrupt painstaking U.S. efforts to reach a nuclear deal with Iran, the State Department said Wednesday.

The Israeli leader opposes any accommodation with Tehran and came to Washington during the campaign to address U.S. lawmakers and denounce the agreement under negotiation as a "bad deal."

Marathon talks to draw up the outlines of a landmark Iran nuclear deal by a March 31 deadline looked set Wednesday to go down to the wire as Tehran played down chances of finishing the job this week.

Iranian Foreign Minister Mohammad Javad Zarif, currently holding talks in Switzerland with U.S. Secretary of State John Kerry, said other foreign ministers from world powers involved in the negotiations were unlikely to join them.

Iran and the U.S. were racing against the clock Sunday to close in on a nuclear deal with US Secretary of State John Kerry saying it was "time to get it done" after 18 months of intense negotiations.

Iran nuclear talks enter their critical end-game Sunday with U.S. Secretary of State John Kerry due to meet his Iranian counterpart in Switzerland as a March 31 deadline looms and a political storm rages in Washington.

After a decade of crisis, 18 months of talks and two missed deadlines, six world powers aim by the end of this month to nail down the outline of a deal with Iran that would put making a nuclear bomb out of Tehran's reach.