Geneva - Iran and six major powers agreed early today on a deal that freezes key parts of Iran's nuclear program in exchange for temporary relief on some economic sanctions, diplomats confirmed.

The deal was reached after four days of marathon bargaining and an 11th-hour intervention by U.S. Secretary of State John Kerry and other foreign ministers from Europe, Russia and China, the sources said.

The agreement, sealed at 3 a.m. signing ceremony in Geneva's Palace of Nations, requires Iran to halt or scale back parts of it nuclear infrastructure, the first such pause in more than a decade.

"We have reached an agreement," Michael Mann, spokesman for European Union foreign policy chief Catherine Ashton said in a Twitter posting that was echoed in a separate posting by Iranian Foreign Minister Mohammad Javad Zarif.

The deal, intended as first step toward a more comprehensive nuclear pact to be completed in six months, freezes or reverses progress at all of Iran's major nuclear facilities, according to Western officials familiar with the details. It halts the installation of new centrifuges and caps the amount and type of enriched uranium that Iran is allowed to produce.

Iran also agreed to halt work on key components of a heavy-water reactor that could some day provide Iran with a source of plutonium. In addition, Iran accepted a dramatic increase in oversight, including daily monitoring by international nuclear inspectors, the officials said.

The concessions not only halt Iran's nuclear advances but also make it virtually impossible for Tehran to build a nuclear weapon without being detected, the officials said. In return, Iran will receive modest sanctions relief and access to some of its frozen accounts overseas, concessions said to value less than $7 billion over the six-month term of the deal. The sanctions would be reinstated if Iran violates the agreement's terms.

The agreement is a long-sought victory for the Obama administration, which from its earliest days made the Iranian nuclear program one of its top foreign policy priorities. The administration, helped by allies as well as Congress, achieved unprecedented success in imposing harsh economic sanctions that cut Iran's vital oil exports in half and decimated the country's currency. But it was hoping to quickly finalize an agreement in the face of threats by Congress to impose additional economic sanctions on Iran.

The deal is also a win for Kerry, who traveled to Geneva twice in two weeks to lend his personal diplomacy to the negotiations.

Still, the agreement is likely to face heavy opposition from key allies - chiefly Israel and Saudi Arabia - as well as congressional skeptics who have demanded much greater concessions from Iran, including the dismantling of its enrichment program.

The marathon discussions with Iran were described by Western diplomats on Saturday as "very difficult" and "intense," and several officials had sought to lower expectations that a resolution could be reached before today, when Kerry and the other foreign ministers were due to depart. Negotiations over the deal had remained snarled late into the late evening Saturday, with the foreign ministers of Britain, France, Germany, China, Russia the European Union and the United States huddled in a hotel conference room.

Several of the diplomats met earlier in the day with Zarif, who told reporters that the parties remained divided on key details of the six-month trial deal.

Kerry, Zarif and Ashton met late Saturday, but the session ended with no announcement of progress. Instead, Iran's deputy foreign minister hardened his country's position.

Although "98 percent" of the deal was done, Iran said it could not accept any agreement that does not recognize what it calls its uranium enrichment rights, Abbas Araghchi told reporters at the time.

"Any agreement without recognizing Iran's right to enrich, practically and verbally, will be unacceptable for Tehran," Araghchi said, according to Reuters.

Araghchi and Zarif have insisted that the deal hinges on international recognition of Iran's right to enrich uranium, a matter of deep national pride.

Western officials have balked at recognizing a legal "right" to uranium enrichment, hoping instead to craft language in the final agreement that acknowledges the right of all countries to pursue nuclear energy for peaceful purposes. Zarif appeared to endorse that approach publicly last week.

The sides also continued to haggle over details of the limited sanctions relief to be offered to Iran in return for scaling back its nuclear program, diplomats said. The agreement frees up a small portion of Iran's overseas currency accounts and eases other trade restrictions.

The most painful sanction, affecting Iran's oil and banking sectors, would remain until the end of the deal's first phase, depending on Iran's willingness to accept permanent curbs on its nuclear program, Western officials said.

Still another obstacle was Iran's partially completed heavy-water reactor in the city of Arak. The agreement freezes construction of the reactor's core, which could, if completed, give Iran a pathway toward obtaining plutonium for nuclear weapons.

Kerry decided to intervene in the talks after negotiators reportedly made progress in overcoming key obstacles to a deal. Kerry joined Russian Foreign Minister Sergei Lavrov, who arrived in Geneva on Friday, as well as British and French counterparts who traveled to Switzerland early Saturday. Diplomats and technical teams from Iran and six major powers had been meeting privately since Wednesday to resolve a number of sticking points.

Close U.S. ally Israel opposes the deal as too generous to an enemy it sees as a mortal threat. Israel is not a party to the talks.

The Obama administration has been unable to reassure Israel or another partner, Saudi Arabia, that the arrangement would make the Middle East safer. The interim deal is meant to build confidence and test Iranian follow-through in preparation for a larger agreement to install strict limits on the Iranian nuclear program in exchange for a gradual repeal of sanctions.

Talks were meant to end Friday. But they were extended as foreign ministers joined lower-level negotiators Saturday for what appeared to be final rounds ahead of what would be a historic agreement to rein in nuclear work that the United States and other nations fear is intended to one day produce atomic weapons.

A spokeswoman for Kerry said he would leave today for other diplomatic meetings. Because Kerry is considered an essential player in finalizing the pact, that set an unofficial deadline to either strike a deal or announce that this round came close but not close enough.

Zarif and Iranian President Hassan Rouhani have pushed for a deal in the past three months, an about-face after years of unproductive talks. These negotiations were the first extended talks between senior U.S. and Iranian diplomats in more than 30 years.