Three months ago, UN Chief Antonio Guterres announced a series of breakthroughs, which are apparently yet to come into effect as a new UNICEF report says at least eight children are still killed or injured in Yemen every day.
Houthi rebels say their leader, Abdul Malek al Houthi, met with Griffiths on Sunday to discuss the implementation of peace deals from December talks with Yemen's internationally recognised government.
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Retired Dutch General Patrick Cammaert chaired the meeting aboard a UN vessel docked off the coast of the flashpoint city of Hudaida.
Yemen's warring parties have failed to pull troops from the country's main port under a month-old UN-sponsored truce, reviving the threat of an all-out assault on Hudaida that risks cutting supply lines and unleashing famine.
Martin Griffiths, UN envoy for Yemen, says, "Such changes in timelines are expected, in light of the facts that the timelines were rather ambitious and we are dealing with a complex situation on the ground."
The Saudi-led coalition has launched a series of air strikes against targets in Yemen's capital Sanaa as the UN struggles to implement a peace deal reached last month.
The exchange of detainees from both Houthi rebels and government forces was agreed in principle as a confidence-building measure ahead of peace talks in Sweden last month.
Saudi and Houthi media said drones belonging to the rebels hit a parade in Lahaj province, killing at least six soldiers. Eyewitnesses say high-ranked officials including Yemen's deputy chief of staff were wounded in the attack.
The rebels have begun to withdraw from the port city, the country's key aid lifeline, under a UN-brokered agreement reached in Sweden earlier this month.
The redeployment follows a ceasefire in the rebel-held city, whose Red Sea port is vital for millions at risk of starvation.
Houthi rebels have blocked roads linking the cities of Taiz and Hudaida, denting efforts to provide aid in a war-torn country where almost 14 million Yemenis are on the brink of famine.
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