NEW YORK (Dispatches) -- The U.S. team negotiating a comprehensive nuclear deal with Iran were "not very optimistic” about the chances of a breakthrough in the talks, a senior U.S. official said Thursday, but bilateral discussions with Iran have been "constructive”.

"Coming into New York, many of us were not very optimistic,” the official said. However, after discussions with Iran Wednesday evening and Thursday morning, "it is clear that everyone has come here to go to work”.

The official said U.S. President Barack Obama is "open to” meeting with Iranian President Hassan Rouhani on the sidelines of the United Nations General Assembly next week but that there was no meeting scheduled.

The official said "the choice is really Iran’s.”

Obama and Rouhani held a 15-minute phone conversation at last year’s annual General Assembly meeting.

Iran is negotiating a nuclear deal with the U.S. and five other powers that would gradually lift sanctions in exchange for major constraints on its nuclear activities. The deadline for an accord is Nov. 24.

Iranian officials have suggested Iran could freeze its current nuclear program but western officials have called for a sharp reduction in enrichment.

"The status quo is not doable for any of us,” the official said.

Asked if there could be an extension of talks beyond Nov. 24 if there was no final nuclear deal, the official said it was "way too early to talk about hypotheticals”.

Iran, which says its nuclear program is purely peaceful, negotiates with the U.S., the U.K., France, Germany, China and Russia.

The official said in the sidelines of the bilateral discussions with Iran on the nuclear program, the two sides touched on Friday’s U.S.-led UN ministerial meeting on ISIL.

The official said the U.S. expects to discuss the challenge posed by ISIL with Iran, as well as other countries, from "time to time”.

The key sticking point is the issue of draconian sanctions on Iran.

Iranian Foreign Minister Mohammad Javad Zarif accused Washington of being "obsessed” with sanctions as a new round of high-stakes bilateral nuclear negotiations opened in New York.

"We are committed to resolving this issue,” Zarif told a U.S. think-tank, as a State Department official confirmed that the two sides had resumed talks here late Wednesday.

But Zarif argued part of the problem blocking a deal was the U.S. "infatuation” with sanctions.

"This deal would require the United States to lift the sanctions, and the reason Congress is objecting to this is that it wants to keep these sanctions,” Zarif told the Council on Foreign Relations.

"Sanctions have become an end in themselves. Sanctions do not serve any purpose,” he said, adding during the time that the Iranian economy has been slapped with Western measures the number of the country’s centrifuges has soared from 200 to 20,000.

"So sanctions have produced, just in normal calculus, 18,800 centrifuges,” he said, joking it is "simple math”.

Zarif, who met Wednesday for a working lunch with European Union foreign policy chief Catherine Ashton, argued: "Iran has shown that we will live up to every agreement that we have.”

A senior State Department official said the bilateral Iranian-U.S. talks would resume again Thursday in New York before the full P5+1 plenary session on Friday.

The talks are expected to last throughout next week on the sidelines of the annual UN General Assembly, and will be joined at some point by foreign ministers.

A new poll released Wednesday found 79% of over 1,000 Iranians surveyed said they would back a deal which even included Iranian assurances never to produce an atomic bomb, but a large majority admitted demands such as dismantling half of Iran’s centrifuges and limiting its nuclear research would be unacceptable.

The poll also revealed deep Iranian skepticism that the West will keep promises to lift the sanctions.

Three-quarters of those surveyed said they believed the U.S. would find some other excuse to impose sanctions, fearing the United States is out to dominate Iran or block its development.

"While the Iranian public is ready to accept taking some confidence building steps, there are obviously some clear limits,” said Ebrahim Mohseni, a senior analyst at the University of Tehran.

President Rouhani "is likely to face a political backlash if he goes farther than the public is ready to support”, he warned.