Griffiths' announcement came at the beginning of the Yemeni peace negotiations outside of the Swedish capital, Stockholm, in a renewed attempt by the UN to end nearly four years of civil war that has pushed the country to the brink of famine.
Representatives of the internationally-recognized Yemeni government, which is backed by a Saudi-led military coalition, flew to Sweden on Wednesday while UN special envoy Martin Griffiths accompanied the Iranian-backed Houthis to Stockholm.
"I am also pleased to announce the signing of an agreement on the exchange of prisoners, detainees, the missing, the forcibly detained and individuals placed under house arrest," Griffiths said at a press conference.
"It will allow thousands of families to be reunited and it is product of very effective, active work from both delegations."
Griffiths hoped the peace talks would result in the resumption of the political process in Yemen.
The Houthis -- rebels belonging to an armed movement that adheres to the Zaidi current of Islam who launched an insurgency against the Yemeni government in 2011 -- did not attend the last round of talks, convened by the UN in September in Geneva.
During that time the rebels reportedly said they were unable or unwilling to travel by air due to concerns that the Saudi-led military coalition would interfere with their flight.
Thursday's peace talks occurred during a time of fighting between Yemen government-supporting coalition forces to take the Houthi-held port city of Hodeida, the main point of entry for food and medicine for Yemen's population.
The head of the Houthi delegation, Mohammed Abdelsalam, vowed to "spare no effort to make a success of the talks" but also warned rebel fighters to remain "vigilant against any attempt at a military escalation".
The Arab country has been locked in a political and military crisis since popular regional uprisings began in 2011, with the Houthis taking control of Yemen's capital Sana'a in September 2014, a move that spurred Saudi Arabia to go to war against the Houthis in 2015 with the support of the United Arab Emirates and other key Saudi allies.
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