MIRED IN MISTRUST

The preliminary accord appeared to arrest a drift towards regional war during which the United States and Israel have both refused to rule out military action against Iranian nuclear sites if the matter cannot be resolved by diplomacy.

Western diplomats, as well as many independent nuclear experts, see it as an important first step that offers a rare chance to finally resolve the nuclear dispute.

Last year's election of a relative moderate, Hassan Rouhani, as Iranian president led to a thaw in ties with the West after years of confrontation and hostile rhetoric.

But Israel, which is believed to have the Middle East's only nuclear arsenal and views a nuclear-armed Iran as an existential threat, has branded the deal a historic mistake as it does not dismantle Tehran's uranium enrichment programme.

Its allies in the US Congress have threatened to impose new sanctions on Iran, even though President Barack Obama has urged them to give diplomacy a chance.

Mark Dubowitz, head of the Foundation for Defense of Democracies in Washington and a proponent of tough sanctions on Iran, said that by providing economic relief, the West will lose future bargaining power.

"The interim deal does nothing over the next 12 months to prevent Iran from proceeding with the nuclear-weapon and ballistic-missile research that are the keys to a deliverable nuclear weapon," he said.

"Ahead of final negotiations, Tehran will be in a stronger position to block peaceful Western efforts to dismantle its military-nuclear programme."

The U.N. nuclear watchdog will play a key role in checking that Iran implements the deal, but its increased access still falls short of what it says it needs to investigate suspicions that Tehran may have worked on designing an atomic bomb.

It is also a far cry from the wide-ranging inspection powers the International Atomic Energy Agency (IAEA) had in Iraq in the 1990s to unearth and dismantle Saddam Hussein's clandestine nuclear programme after the first Gulf war.

"The accord gives the powers and Iran plenty of flexibility in going about reducing Iran's nuclear threat to a level the world will accept," said proliferation expert Mark Hibbs of the Carnegie Endowment think-tank.

"But it hasn't spelled out how they will work with the IAEA to resolve allegations that Iran has been working on nuclear weapons."

Reaching the final accord will mean overcoming decades of deep-seated mistrust between Iran and the West, and politicians on both sides have warned it will be hugely challenging.

Obama said this month he had "no illusions" about how hard it would be to secure a comprehensive agreement.