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The UN Special Envoy to Yemen Martin Griffiths has arrived in Sanaa on an unannounced visit, a source at the city's airport revealed.
"Griffiths arrived in Sanaa accompanied by his delegation, on Tuesday morning," the source said.
The visit comes as the Yemen's Houthis, also known as Ansar Allah, released 290 captives on Monday, as reported by the International Committee of the Red Cross.
The group's prisoner affairs committee also revealed the same day that it was offering to release 350 hostages that were in the lists included in the prisoner swap deal that had been agreed upon in Stockholm in December.
Griffiths welcomed the move, saying it would hopefully lead to further progress on the prisoner swap deal.
The Stockholm Agreement is part of UN Security Council resolution urging a 30-day deployment of a United Nations team to Yemen to help monitor the armistice in Al Hudaydah during the UN-led consultations in Sweden.
According to the resolution, the troops of both sides of the conflict were supposed to withdraw from the area, exchange prisoners of war and establish humanitarian corridors in Al-Hudaydah.
Yemen has been ravaged by an armed conflict between government forces, led by President Abdrabuh Mansour Hadi, and the Houthi movement. A Saudi-led coalition has been carrying out airstrikes against the Houthis at Hadi's request since March 2015.