| LAUSANNE, Switzerland
LAUSANNE, Switzerland Six world powers and Iran negotiated past a March 31 deadline into the wee hours of Wednesday, struggling to conclude an outline accord on Tehran's nuclear program in the face of a U.S. threat to abandon the talks.
With Iran asserting its "nuclear rights", the talks in the Swiss city of Lausanne entered a seventh day, bogged down this time on the issues of nuclear research, the lifting of U.N. sanctions and their restoration if Iran breached the agreement.
Officials cautioned that any accord would be fragile and incomplete, but the U.S. State Department gave the go-ahead for talks to go past a self-imposed midnight deadline.
"We've made enough progress in the last days to merit staying until Wednesday," acting spokeswoman Marie Harf said in a statement. "There are several difficult issues still remaining."
The six powers - the United States, Britain, France, Germany, Russia and China - aim to stop Iran from gaining the capacity to develop a nuclear bomb in exchange for easing international sanctions that are crippling its economy.
The outline political accord is meant to lay the foundation for a final settlement of the long-running nuclear dispute by June 30 - another self-imposed deadline, but one that Western powers have said they do not want to extend.
A senior Iranian negotiator said Tehran was willing to negotiate until the deadlock was resolved.
"Iran does not want a nuclear deal just for the sake of having a deal, and a final deal should guarantee the Iranian nation's nuclear rights," the negotiator, Hamid Baidinejad, told reporters.
White House spokesman Josh Earnest said in Washington that U.S. negotiators would not wait until June 30 to walk away from the talks if they could not reach a preliminary political agreement.
"If we're not able to reach a political agreement, then we're not going to wait ... until June 30 to walk away," he said.
A German delegation source said "it remains an open question whether we will succeed", adding that it was "too early to think about stopping the clock, (though that) may perhaps prove necessary".
French Foreign Minister Laurent Fabius told reporters in Lausanne that "we are moving forward, but it's complicated."
Disagreements on enrichment research and the pace of lifting sanctions threatened to scupper a deal that could end the 12-year-old standoff and reduce the risk of another Middle East war. Iran says its nuclear program is peaceful.
For days officials have been trying to agree on a brief document of several pages outlining headline numbers to form the basis of a future agreement. Parts of any understanding reached by the parties will likely remain confidential, though they will likely issue a statement if a deal is reached.
It is possible they would not agree on anything. "We are preparing for both scenarios," a Western diplomat said.
Speaking in Berlin with German Chancellor Angela Merkel, French President Francois Hollande said it would be better to have no deal than a bad deal.
An agreement would almost certainly lift sanctions only in stages, deferring even a partial return of Iranian crude 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. Brent crude dropped toward $55 a barrel on Tuesday as negotiations proceeded.
Officials from both sides said the main sticking points were the removal of the U.N. sanctions, their reversibility and Iranian demands for the right to unfettered research and development into advanced nuclear centrifuges after the first 10 years of the agreement expires.
Officials said Iran was still demanding the lifting of all U.N. sanctions and that they not be automatically reinstated without further negotiations. Officials have said Britain, France and the United States want any removal of U.N. sanctions to be automatically reversible, but the Russians dislike this because it would weaken their veto power over the deal.
The six powers want more than a 10-year suspension of Iran's most sensitive nuclear work. Their goal is to find a way to ensure that for at least the next 10 years Iran is at least one year away from being able to produce enough fissile material for an atomic weapon.
Iran said the main issue was lifting sanctions quickly. "There will be no agreement if the sanctions issue cannot be resolved," Majid Takhteravanchi, an Iranian negotiator, told Iran's Pars news agency.
Russian Foreign Minister Sergei Lavrov said he believed there was a good chance of success "if none of the parties raise the stakes at the last minute".
The U.S. Congress has warned it will consider imposing new sanctions on Iran if there is no agreement this week, giving a sense of urgency to the talks.
U.S. President Barack Obama has threatened to veto any sanctions moves by the Republican-dominated Congress.
In Jerusalem, Prime Minister Benjamin Netanyahu reiterated Israel's concern that an agreement would fall short of guaranteeing its safety.
The framework agreement would leave Iran with the capability to develop a nuclear weapon in under a year, said Netanyahu, whose country is believed to have the Middle East's only nuclear arsenal.
(Additional reporting by Stephanie Nebehay; Editing by Howard Goller)