Saudi Arabia is leading a military coalition in helping President Hadi regain control of the country. The rival Houthi forces are reportedly supported by Iran.

Griffiths said military confrontation and airstrikes have continued in many parts of Yemen, with "disturbing reports" of civilian casualties.

He was also worried by unconfirmed reports that movement of forces in Yemen is on the increase and that intensive military operations in Hudaydah may be forthcoming.

"Our concern is that any of these developments may in a stroke, take peace off the table," he warned. "We all need urgently and creatively to find ways to diminish the chances of these game-changing events, upsetting and derailing the hopes of the great majority of Yemenis."

"There is a saying that the darkest hour is the one before dawn. Let us hope this is the case," said Griffiths.