Iran, six world powers clinch breakthrough nuclear deal

* Deal halts Iran nuclear progress, U.S. says

* Agreement follows marathon talks in Geneva

* "This is only a first step" - Iran minister

* Most sanctions to remain in place -Kerry (Adds quotes, details on agreement, analyst)

By Parisa Hafezi and Justyna Pawlak

GENEVA, Nov 24 (Reuters) - Iran and six world powers reached a breakthrough deal early on Sunday to curb Tehran's nuclear programme in exchange for limited sanctions relief, in what could be the first sign of an emerging rapprochement between the Islamic state and the West.

Aimed at ending a dangerous standoff, the agreement between Iran and the United States, France, Germany, Britain, China and Russia was nailed down after more than four days of tortuous negotiations in the Swiss city of Geneva.

Halting Iran's most sensitive nuclear work, it was designed as a package of confidence-building steps to ease decades of tensions and confrontation and banish the spectre of a Middle East war over Tehran's nuclear aspirations.

European Union foreign policy chief Catherine Ashton, who has been coordinating talks with Iran on behalf of the major powers, said it created time and space for talks aimed at reaching a comprehensive solution to the dispute.

"This is only a first step," Iranian Foreign Minister Mohammad Javad Zarif told a news conference. "We need to start moving in the direction of restoring confidence, a direction in which we have managed to move against in the past."

In Washington, U.S. President Barack Obama said that if Iran did not meet its commitments during a six-month period, the United States would turn off sanctions relief and "ratchet up the pressure."

But Israeli Prime Minister Benjamin Netanhayu's government denounced the agreement as "a bad deal" that Israel did not regard itself as bound by.

Before Sunday's agreement, Israel, believed to be the Middle East's only nuclear power, said the deal being offered would give Iran more time to master nuclear technology and amass potential bomb fuel.

The West fears that Iran has been seeking to develop a nuclear weapons capability. The Islamic Republic denies that, saying its nuclear programme is a peaceful energy project.

The United States said the agreement halted progress on Iran's nuclear programme, including construction of the Arak research reactor, which is of special concern for the West as it can yield potential bomb material.

It would neutralise Iran's stockpile of uranium refined to a fissile concentration of 20 percent, which is a close step away from the level needed for weapons, and calls for intrusive U.N. nuclear inspections, a senior U.S. official said.