VIENNA - US Secretary of State John Kerry is to join the Iranian nuclear negotiations Thursday evening, a US spokesperson said.

"Secretary Kerry will travel from Paris, France to Vienna, Austria this evening to check in on the Iran nuclear negotiations.

His date of departure from Vienna is not yet determined," spokesperson Jen Psaki said in an email.

Iran and the P5+1 group(five permanent members of the UN Security Council plus Germany) are heading to a historical agreement to end the decade-old stand-off between Iran and the West, before the Nov. 24 deadline.

As time is running out, there are still gaps over the scale of Iran's nuclear program and the western sanction relief.

Kerry decided to join the negotiations after exchanging views with his British, French and Omani counterparts.

Negotiation teams from Iran and the P5+1 group have been discussing a possible comprehensive solution for Iran's nuclear plan since this Tuesday.

It's still unclear if negotiators have worked out a different path to the final deal.

Experts believe Iran and the west won't walk away as the stakes are too high, and it could be the best opportunity for both sides to end the stand-off.

But it seems that time is limited for sides to work out a comprehensive deal in four days with remaining disagreements.

"I'm not optimistic that we can get everything done by Monday," said Philip Hammond, Britain's Secretary of State for Foreign and Commonwealth Affairs, during his trip to Latvia on Wednesday. "But I think if we make some significant movement we may be able to find a way of extending the deadline to allow us to get to the final deal."

"If, because of excessive demands," an agreement was not reached, "the world will understand that the Islamic Republic sought a solution, a compromise and a constructive agreement," Mohammad Javad Zarif, Iran's Minister of Foreign Affairs, told the Iranian media after arriving in Vienna airport on Tuesday.