By John Irish, Parisa Hafezi and Louis Charbonneau
LAUSANNE, Switzerland (Reuters) - The prospects for a preliminary agreement between Iran and world powers on Tehran's nuclear programme were finely balanced between success and collapse on Thursday, diplomats said.
The negotiations, aimed at blocking Iran's capacity to build a nuclear bomb in exchange for the lifting of sanctions on the country, are bogged down over details of the accord even though the broad outlines of an agreement have been reached.
Ploughing on beyond a March 31 deadline, negotiators talked until 6 a.m. (0400 GMT) on Thursday in the Swiss city of Lausanne, breaking off for three hours to rest.
Accounts differed on the extent of progress, with Iran's Foreign Minister Mohamad Javad Zarif calling it "significant" but a Western official describing it as "limited".
"The result is far from being imminent," said one Western diplomat.
Another diplomat expressed the hope there would be a breakthrough in the coming hours, though there were no signs one was forthcoming.
Another diplomat said there were still key disagreements on the core issues of Iran's nuclear centrifuge research and U.N. sanctions, but that U.S. Secretary of State John Kerry was determined to leave Lausanne with some form of detailed deal.
Zarif told reporters if all sides could agree on the outlines then there would be a statement.
"I wish I had time to eat," a tired Kerry told reporters.
Ministers and experts shuffled from meeting to meeting throughout Thursday as talks entered their eighth day.
Kerry and German Foreign Minister Frank-Walter Steinmeier said they would stay at least until Thursday in an effort to seal the "political" agreement, a milestone towards a final pact due by the end of June. It was unclear whether the talks would run into Friday.
UNDER PRESSURE
Six world powers - the United States, Britain, France, Germany, Russia and China - aim to stop Iran from gaining the capacity to develop a nuclear bomb. Tehran wants to lift international sanctions that have crippled its economy, while preserving its right to nuclear technology for peaceful purposes.
The powers and Iran said they had moved closer but both sides accused the other of refusing to offer proposals that would break the deadlock.
The talks - the culmination of a 12-year process - have become hung up on the issues of Iran's nuclear centrifuge research, details on the lifting of U.N. sanctions, and how they would be re-imposed if Iran breached the agreement.
All sides are under pressure not to go home empty handed, but Washington reiterated on Wednesday it was willing to walk away if the sides could not agree on a preliminary framework. White House spokesman Josh Earnest told reporters in Washington: "the time has come for Iran to make some decisions."
There were also signs that U.S. President Barack Obama was coming under renewed pressure to walk away.
Republican senators John McCain and Lindsey Graham predicted the talks would not end well.
"Any hope that a nuclear deal will lead Iran to abandon its decades-old pursuit of regional dominance through violence and terror is simply delusional," they said in a joint statement.
The talks represent the biggest chance of rapprochement between Iran and the United States since the Iranian revolution in 1979 but face scepticism from conservatives in both nations' capitals. Washington's allies in the region, especially Israel and Saudi Arabia, are also deeply wary of any deal.
Even if there is a preliminary deal, it will be fragile and incomplete. But Iran has repeatedly expressed optimism that an initial agreement was within reach, as has Russia, which with China is closest to Iran among the powers.
A key goal of the talks for Washington is to impose conditions on Iran that would increase the "breakout time" Tehran would need to develop a nuclear weapon if it should decide to pursue one.
The United States is also trying to ease fears in Israel and Sunni Gulf Arab states that an atomic deal would leave the door open to Tehran gaining a nuclear weapon, or would ease political pressure on it, giving it more space to back Arab Shi'ite proxies in the region.
Oman, an intermediary when Tehran and Washington launched secret talks on a possible nuclear deal in 2013, said failure to achieve a deal would mean catastrophe for the region.
"There are those who prefer peace, this is why there are negotiations between the 5+1 and Iran," Foreign Minister Yusuf bin Alawi told Reuters in Muscat. "Those who prefer wars - they should be willing to accept losses. Heavy losses. Catastrophe."
Israeli Strategic Affairs Minister Yuval Steinitz told Reuters he believed there would be some kind of declaration as no one wanted to admit failure.
"It will be quite a joke. It's a quite a farce, what's happening there."
An agreement would almost certainly lift sanctions only in stages, deferring even a partial return of Iranian crude oil exports until at least 2016. Sanctions have halved Iran's oil exports to just over 1 million barrels per day since 2012 when oil and financial sanctions hit Iran.
Oil futures fell on Thursday, retreating from big gains in the previous session, on the prospect of a deal and a possible increase in Iran's crude exports.
(Additional reporting by Patricia Zengerle in Washington; Editing by Janet McBride)
