French Foreign Minister Laurent Fabius, US State Secretary John Kerry, former European Union (EU) foreign policy chief Catherine Ashton, British Foreign Secretary Philip Hammond and German Foreign Minister Frank-Walter Steinmeier (L-R) meet for dinner at the residence of the British ambassador in Vienna November 23, 2014. [Photo/Agencies]

VIENNA - More time might be needed in order to seek a comprehensive agreement as the issues being discussed in the Iran nuclear talks are complicated, a diplomat closed to the talks told Xinhua on Sunday.

The diploma told Xinhua sides are still focusing on reaching a deal by Monday, the deadline, but since the issues are complicated, the negotiators might need more time to bridge gaps.

Earlier, an Iranian official told Iranian Students News Agency (ISNA) that it is impossible to agree on a comprehensive deal by November 24 due to time limit and lots of details yet to be discussed.

US Secretary of State John Kerry on Saturday said big gaps remain between sides.

Kelsey Davenport of the Arms Control Association said if a deal is not struck before January, it could be hard to prevent the new congress from taking actions that would hurt the talks, but she said a brief extension is possible.

Negotiators are aimed to reach a comprehensive settlement at talks in Vienna that would curb Iran's atomic activities, in return for a phasing out of sanctions that have severely hurt the gulf country's oil-dependent economy.