Rimbo, Sweden - Yemen’s warring parties held the first direct talks in UN-led peace efforts in Sweden on Sunday to iron out a prisoner swap, one of several confidence-building measures intended to help launch a political process to end nearly four years of conflict.

Since talks began on Thursday, UN officials have been shuttling between delegations from the Iranian-aligned Al Houthi militia and the government of President Abd Rabbu Mansour Hadi which is backed by a Saudi-led military coalition.

The war in Yemen has killed tens of thousands of people and caused a major humanitarian disaster.

Mediator Martin Griffiths opened the new round due to last until December 13 by announcing a deal to release thousands of prisoners. The two sides met in a renovated castle outside Stockholm on Sunday to discuss implementation of the deal.

The team from Hadi’s government initially refused to enter the room, saying Al Houthis needed to include more senior delegates, but the meeting went ahead, delegates said.

“We are very optimistic about having a breakthrough on the prisoners issue ... we have exchanged some lists in the past but each side needs to update them,” said Askar Ahmad Zayl, a delegate from Hadi’s government.

The parties have yet to agree on trickier issues such as re-opening Sanaa airport and a truce in the port city of Hodeida, both held by Al Houthis, both further confidence-building measures that are the focus of the talks in addition to a framework for negotiations.

“We have three or four days. If we end up without any agreement then this round has failed,” Al Houthi’s main negotiator Mohammad Abdul Salam told reporters.

“But if we have a draft on some general framework, the reopening of Sana’a airport, the prisoners release, keeping the central bank neutral and a de-escalation in Hodeida ... then this will be a good step to hold another round in one or two months.” Griffiths praised the parties’ “positive spirit” in engaging constructively and urged for calm on the ground in Yemen, where sporadic battles have continued in the Red Sea port of Hodeida, a lifeline for millions, and other parts of the country.