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'Difficult issues' remain as Iran nuclear talks go beyond deadline

International talks to reach a preliminary agreement on Iran's nuclear program have run past their Tuesday deadline and into another day, deepening uncertainty over whether a deal will be reached.

The negotiations are seen as a crucial effort to use diplomatic means to try to ensure Tehran isn't able to rapidly move toward building a nuclear bomb.

\"We've made enough progress in the last days to merit staying until Wednesday. There are several difficult issues still remaining,\" U.S. State Department spokeswoman Marie Harf said.

White House press secretary Josh Earnest said the talks in Lausanne, Switzerland, would continue another day as \"long as the conversations continue to be productive.\"

The negotiating teams worked late Tuesday as their self-imposed deadline approached, but more time was deemed necessary to try to strike a framework agreement -- a political understanding on the main principles of a deal.

Trying to overcome decades of mistrust

How long talks would continue was unclear.

The French, Russian and Chinese Foreign Ministers left the venue of the talks Wednesday morning, leaving lower ranking officials to represent them. But the ministers could return if enough progress were made to warrant further discussions.

The other nations taking part in the talks are the United Kingdom and Germany.

But the real onus is on Washington and Tehran to overcome decades of deep mistrust to reach a deal.

\"You're talking here about an institutionalized relationship of hostility between the United States and Iran,\" said professor of Middle East studies at The London School of Economics.

Final deadline end of June

For Iran, which insists its nuclear program is for peaceful purposes, a deal would mean relief from punishing economic sanctions.

For the West, it would offer hope of improved relations with Tehran without the destabilizing threat of an Iranian nuclear bomb in the pipeline.

Hamid Ba'idinejad from Iran's Foreign Ministry said Tuesday that there were no \"artificial\" deadlines and that a deal would be reached when each issue had been resolved.

Mohammad Marandi, a professor for North American studies at Tehran University, told CNN that Iran officials feel they've already compromised a lot in the talks.

\"They feel that they've gone as far as they possibly can and that it's for the Americans right now to make a move,\" he said.

The framework deal that negotiators are pursuing in Lausanne wouldn't be the end of the story. Talks would then move on to fleshing out the framework into a comprehensive agreement covering all the technical details.

That has to be reached by the end of June, the final deadline for the tortuous negotiation process that has taken years to get to this stage.

Even if a preliminary deal is struck in Lausanne, it seems that some of the trickiest issues could end up being pushed into the final phase of talks.

Israeli opposition

The main sticking points at the moment are believed to be the pace at which U.N. sanctions on Iran will be lifted, how much nuclear research and development Iran will be able to maintain and whether Iran will ship its stockpile of enriched uranium out of the country for reprocessing into a safer form.

Even if those points are resolved to the satisfaction of all sides, any agreement is expected to come under attack on multiple fronts.

U.S. President Barack Obama is likely to face a stern challenge selling any deal to Congress, while hardliners in Iran will probably denounce it for being too harsh.

Israel, a key U.S. ally that considers Iran to be an existential threat, has repeatedly said it believes the deal likely to emerge from the talks wouldn't stop Tehran from developing a nuclear bomb.

Reiterating that stance, Israeli Prime Minister Benjamin Netanyahu said Tuesday that \"the greatest threat to our security and safety and our future is Iranians' attempt to become nuclear.\"

\"And the agreement that is being formed in Lausanne,\" he said, \"is paving the road to that result.\"

U.S.: 'The military option will remain on the table'

Obama administration officials say that any agreement would involve heavy monitoring of Iran's nuclear activities. They insist no deal would be better than signing up to a bad deal.

But if no agreement is reached and the talks fall apart, the potential consequences are deeply unsettling.

Further sanctions on Iran would most likely follow. Israel argues that could eventually force Tehran back to the negotiating table to settle for a tougher deal.

But Obama administration officials say they fear Iran will redouble efforts to advance its nuclear program without any meaningful international inspections.

Iranian progress toward a nuclear weapon could spark an arms race in the Middle East and talk of military strikes from the United States or Israel.

\"The military option will remain on the table,\" U.S. Defense Secretary Ashton Carter told NBC's \"Today\" show. \"If there is a good agreement to have, obviously it's worth waiting for and completing the negotiations.\"

CNN's Elise Labott reported from Lausanne, and Jethro Mullen reported and wrote from Hong Kong. Jim Sciutto, Steve Almasy, Greg Botelho, Nimet Kirac and Catherine E. Shoichet contributed to this report.

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