UN Secretary-General Special Envoy Ismail Ould Cheikh Ahmed (C) open with delegations from Sanaa at the Yemen peace talks in Switzerland December 15, 2015. [Photo/Agencies]

Islamist militants are also involved in the conflict.

An earlier round of UN-backed indirect talks in Geneva in June ended without an agreement, with both sides blaming each other for their collapse. Unlike the previous round, the current session opened with an agenda being agreed and with senior delegates meeting face-to-face.

The main task for the negotiations will be agreeing on how to implement a UN Security Council Resolution in April that called on the Houthi movement to quit Sanaa and other cities.

Yemeni Prime Minister Khaled Bahah said in Qatar that the purpose of the talks was to win back the state seized by the Houthis. The option of using force to achieve that remained on the table, he said.

"Despite the optimism, and based on our experience, the talks won't be easy," Bahah said, according to the Qatari state news agency QNA.

"We are seeking to reach peaceful solutions but the stick will remain to achieve what could not be achieved in the talks."