Yemen's Houthi rebels have unilaterally released 290 detainees as part of a UN peace initiative, the International Committee of the Red Cross says.
Those freed include 42 survivors of an air strike on a prison this month that killed more than 100 people.
The attack was carried out by a Saudi-led coalition backing the Yemeni government in the country's civil war.
The UN special envoy for Yemen said he hoped the Houthis' step would lead to further releases by both sides.
A prisoner swap was one of three elements of an agreement between the warring parties that was brokered by the UN in Sweden's capital, Stockholm, in December.
The head of the Houthi committee for prisoner-of-war affairs, Abdul Qader al-Murtada, said on Monday the rebels would free 350 detainees, including three unidentified Saudi nationals.
"Our initiative proves our credibility in implementing the Sweden agreement and we call on the other party to take a comparable step," he added.
Later, the ICRC said it and the UN had facilitated the release of 290 detainees.
The humanitarian organisation gave no details about the identities of the detainees, besides saying that they included survivors of the Dhamar attack.
"The release is a positive step that will hopefully revive the release, transfer and repatriation of conflict-related detainees as per the Stockholm agreement," it said.
UN special envoy Martin Griffiths also welcomed the initiative and called on all parties to ensure the safe return of the released detainees to their homes.
The UN says the fighting in Yemen has left at least 7,000 civilians dead. But monitors believe the death toll is far higher. The US-based Armed Conflict Location and Event Data Project (ACLED) estimates that more than 90,000 civilians and combatants have been killed.
The civil war has also triggered the world's worst humanitarian disaster, with thousands of civilians dying from preventable causes, including malnutrition, disease and poor health.