WASHINGTON: The United States plus five world powers reached a landmark deal with Iran on Sunday to curtail the Persian country's purported march towards nuclear weapons.

The agreement, when fully realized, has the potential to dramatically alter the geo-political landscape of the Middle-East, Gulf, and South Asia, affecting the strategic outlook and orientation of major countries from Israel to India and in between.

Under the first phase of the agreement, clinched in a 3am signing ceremony in Geneva, Iran will stop enriching uranium beyond five per cent, effectively giving up the higher levels of enrichment needed to produce fissile material for nuclear weapons. It will also divert or convert its stockpile of 20 per cent enriched uranium into an oxide form so it cannot be used for military purposes.

Iran will also not install any new centrifuges nor start up any that are not already in operation or build new enrichment facility, while submitting to daily international inspections that will make it almost impossible for it to work towards making nuclear weapons.

In return, Iran will get to keep its existing centrifuges, be able to enrich uranium below five per cent for civilian nuclear uses, and receive relief from crippling US-led sanctions (including getting some revenues seized by past sanctions) for the next six months, during which a more detailed, longer term agreement will be negotiated.

At a broader level, it will begin the process of recasting strategic alignments in the region. Untrusting Israel, haunted by an existential crisis that comes from a (mutual) pathological fear of a nuclear-armed rival, straightaway rejected the deal, suggesting US and its allies had been suckered by Teheran. Sunni-dominated Saudi Arabia, which fears its cozy equation with Washington being eclipsed by a Shia-dominated Iran returning to the US sphere of influence, also lashed out at the agreement.

Nearer home, the US-Iranian detente provides an exit route for the United States from landlocked Afghanistan while reducing its dependence on extremist Pakistan, which is extracting a ransom for the 2014 drawdown from Afghanistan.

It will also come as a big relief for India, which has had to do juggle and balance four aspects — its growing strategic partnership with the US, its strong military relationship with Israel, its economic and social investments in Afghanistan, and its civilizational ties with the Persian power. An Indian-built road from the Afghan border town of Zaranj to the Iranian port of Charbahar suddenly comes into play.

Eventually, India may also be able to resume normal trade relations with Iran, which the US-led sanctions had put a crimp on.

The US-led deal is interim in nature and there is much that can go wrong in the six months during which the concerned parties will negotiate a more comprehensive deal. For now though, both sides exulted on having broken new ground, and both claimed to have gained from the accord, effectively pointing to a win-win situation.

"It is important that we all of us see the opportunity to end an unnecessary crisis and open new horizons based on respect, based on the rights of the Iranian people and removing any doubts about the exclusively peaceful nature of Iran's nuclear program," Iranian foreign minister Mohammed Javad Zarif, who played a key role in the talks, told reporters. "This is a process of attempting to restore confidence."

President Obama, speaking from the State Dining Room in the White House, said diplomacy "opened up a new path toward a world that is more secure — a future in which we can verify that Iran's nuclear program is peaceful and that it cannot build a nuclear weapon."

But disquiet and unease were evident in the reactions from Israel and Saudi Arabia, although Obama pledged that as negotiations go forward, US will retain steadfast in its commitments to "friends and allies — particularly Israel and our Gulf partners, who have good reason to be skeptical about Iran's intentions."

That skepticism was aired openly. "What was concluded in Geneva last night is not a historic agreement, it's a historic mistake," Israeli Prime Minister Benjamin Netanyahu told reporters. "It's not made the world a safer place. Like the agreement with North Korea in 2005, this agreement has made the world a much more dangerous place."

Netanyahu maintained that Iran would be "taking only cosmetic steps which it could reverse easily within a few weeks, and in return, sanctions that took years to put in place are going to be eased."

But US interlocutors appeared confident that they had the lock on Iran's route to a nuclear weapon. "It will make our partners in the region safer. It will make our ally Israel safer," secretary of state John Kerry , who led the US-allied talks, said.

(With inputs from agencies)