0619 GMT April 18, 2019
Agreements reached in December between the Houthi movement and the Saudi-backed former government were the first significant breakthrough in nearly five years of conflict which has killed tens of thousands of people. But little progress has been made on the trust-building measures, risking the unraveling of peace efforts, Reuters reported.
"The initial timelines were rather ambitious. We are dealing with a complex situation on the ground," Griffiths said on Twitter.
The Houthis control Hodeida, the main entry point for the bulk of Yemen's commercial and aid imports and a lifeline for millions of starving Yemenis.
Saudi-led coalition troops are massed on its outskirts. The warring sides disagree over who should control the city and port after forces withdraw.
The truce in Hodeida has largely been respected since coming into force a month ago, but skirmishes continue. Troops have not yet pulled out, missing a Jan. 7 target, and residents and aid workers have said that barricades, trenches and roadblocks have been reinforced.
While rhetoric by all sides has becoming increasingly bellicose as fighting escalates in other parts of the Arabian Peninsula nation, Griffiths said he remained optimistic.
"More than any time in the past, there is a political will demonstrated by all parties to put an end to this conflict," he said. "What we need to see now is the implementation of the provisions of the agreement, fully and rapidly."
A troop withdrawal from Hodeida would mark a turning point, he said.
Western nations, some of which supply the coalition with arms and intelligence, are pressing for an end to the conflict.