WASHINGTON — Secretary of State John F. Kerry and other top Western diplomats flew to Geneva on Friday in the hope of completing an accord to freeze Iran's nuclear program, but the high-stakes diplomatic push ran into obstacles that dimmed prospects for a quick resolution after a decade-long stalemate.

Instead of the expected signing ceremony Saturday, disappointed diplomats said they expect the talks to spill at least into next week.

Diplomats declined to say why the deal has stalled, but the two sides are known to disagree about Iran's construction of a plutonium reactor, a possible route to a nuclear bomb. The international community wants Iran to agree not to activate the reactor while the interim deal is in place. But they also may be pressing for more restrictions.

A meeting Friday between Kerry, Iranian Foreign Minister Mohammad Javad Zarif and European Union Foreign Policy Chief Catherine Ashton lasted until 11:30 p.m., aides said, but failed to break the deadlock.

"The negotiations have reached [a] critical, very sensitive situation, and it needs decisions at higher levels," said Abbas Araghchi, Iran's deputy foreign minister and chief negotiator. An apparent diplomatic breakthrough in 2009 collapsed after leaders in Tehran weighed in negatively.

The latest plan was expected to lay out a series of reciprocal steps intended to pave the way for further negotiations. The ultimate goal is a comprehensive deal that ensures Iran will give up its nuclear ambitions in exchange for the United States and other governments lifting sanctions that have strangled its economy.

Hoping to make an official announcement, Kerry was joined Friday by his counterparts from Britain, France and Germany. The foreign ministers of Russia and China were expected to arrive Saturday. The five permanent members of the United Nations Security Council, plus Germany, have united as a diplomatic bloc in the negotiations with Iran.

Hawkish critics in Congress, in Israel and elsewhere already have begun taking aim at the proposed multistaged plan, warning that even a modest "first step" agreement under consideration would relieve too much pressure on Iran if it did not require a halt to all uranium enrichment during the expected six months of final negotiations.

"Any deal that does not require a full and complete halting of the Iranian nuclear program is worse than no deal at all," House Majority Leader Eric Cantor (R-Va.) said in a statement Friday, echoing comments from several other key lawmakers and Israeli Prime Minister Benjamin Netanyahu.

Rep. Ed Royce (R-Calif.), chairman of the House Foreign Affairs Committee, said he would convene a hearing Wednesday. "Instead of toughening sanctions to get meaningful and lasting concessions, the Obama administration looks to be settling for interim and reversible steps," he said.

Rep. Eliot Engel (D-N.Y.), ranking member of the Foreign Affairs Committee and a key advocate of sanctions, said he was "deeply troubled" by reports that the deal wouldn't halt all enrichment.

Some lawmakers raised the possibility that Congress could slap even harsher sanctions on Iran during the next phase of the talks. Administration officials fear that could drive Iran from the table and end any chance of a peaceful resolution.

White House officials scrambled Friday to prevent this narrative from taking hold before the deal is even finished. They insisted the agreement would bar Iran from making any progress toward bomb-making capability while negotiations are underway, and that President Obama has not taken

a military option off the table.

Obama telephoned Netanyahu after the Israeli leader spoke out against the prospective accord, telling reporters in Jerusalem that "the Iranians are walking around very satisfied in Geneva as well they should be.... Iran got the deal of the century and the international community got a bad deal."

In a brief statement, the White House said Obama updated Netanyahu on the negotiations "and underscored his strong commitment to preventing Iran from obtaining a nuclear weapon."

Though many details of the proposed deal are unknown, some elements have come into view. Iran would be required to halt production of medium-enriched uranium, which can easily be converted into bomb fuel, and to disable much of its existing stockpile.

It also would be required to stop using its most sophisticated centrifuges, the high-speed IR2M model, according to diplomats who spoke on condition of anonymity in discussing the sensitive negotiations.

These steps are intended to keep Iran from producing uranium that could be quickly upgraded into fissile fuel for a bomb, a scenario that diplomats call a "breakout."

In exchange, diplomats say, Iran would be given access to some of the billions of dollars that the West has frozen in overseas bank accounts, foreign exchange holdings that are vital for its battered economy. It also would be granted temporary relief from some secondary sanctions, such as using gold and other precious metals in international trades.

But the proposed deal, diplomats say, doesn't prohibit Iran from continuing to produce low-enriched uranium, which is suitable for reactors, medical research and other industrial uses. The purity of the material would need to be boosted substantially to become weapons-grade fissile material.

Critics demanding a total halt to enrichment contend that Iran could secretly make advances toward a nuclear weapon. But defenders of the administration's approach say that such enrichment does not move Iran toward a breakout capacity, and argue that it is unrealistic to expect that the Islamic Republic, where there is strong support for the nuclear program, would agree to a complete halt.

Some congressional aides and analysts said that though members of Congress may not try to quickly ram through tough sanctions, they are likely to maintain their pressure during the six months of negotiations and intervene if they believe the United States and its partners are being duped by Iran.

"They want no relaxation of pressure here at all," said Michael Singh, who is managing director of the Washington Institute for Near East Policy and a former national security aide in the George W. Bush White House. "They're going to try to hold the feet of both the administration and Iran to the fire as this goes on."

paul.richter@latimes.com

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