In spite of the high-level discussion on Sunday between Iran and the United States as the deadline for a nuclear agreement fast approaches, a big gap remains, warned U.S. President Barack Obama.

"Are we going to be able to close this final gap so that [Iran] can re-enter the international community, sanctions can be slowly reduced and we have verifiable, lock-tight assurances that they can't develop a nuclear weapon?" Obama said in a CBS News interview.

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Due to the large gap to convert the temporary agreement into a long-term one by Nov. 24, the chances of meeting the deadline are becoming slimmer, he said.

U.S. Secretary of State John Kerry discussed in Omani capital of Muscat with his Iranian counterpart, Foreign Minister Mohammad Javad Zarif, amid stronger pressure from both sides to conclude the long-running negotiations. European Union Foreign Policy Chief Catherine Ashton was also present in the meeting, said to be projected to run until Monday.

Iran said it will not abandon its nuclear rights, but is committed to the ongoing talks, said Ali Akbay Velayati, the top aide of Iranian Supreme Leader Ayatollah Ali Khamenei.

The Sunday negotiations at a luxury hotel started at 11:30 a.m. and broke at 2:30 p.m. for lunch and consultations. The three went back to their discussions before 6 p.m., reports Reuters.

The key issue is how many uranium-enriching centrifuges Iran would be allowed to retain in exchange for lifting of the sanction and strict inspections of its nuclear facilities. Iran still insists on an industrial grade enrichment capability over its current one.

Meanwhile, Kerry downplayed the link between the nuclear discussions and the secret letter that Obama wrote to Khameini that tackled fighting a common enemy - the Islamic State - even if their reasons for doing so are different.

Failure to convert the interim agreement into a long-term deal could result in the U.S. Congress pushing for fresh sanctions against Iran, while Iranian legislators want any final agreement to be ratified by the country's Parliament.

TagsIran nuclear program, John Kerry, Barack Obama, Iranian Foreign Minister Mohammad Javad Zarif