Griffiths said the rate of air strikes fell by 80 percent during that period, which he said was "perhaps an even more important sign that something is changing in Yemen."

"In recent weeks, there have been entire 48-hour periods without air strikes for the first time since the conflict began," he said via a video link to the UN headquarters.

"We call this de-escalation, a reduction in the tempo of the war, and perhaps a move towards an overall ceasefire in Yemen," he said.

In Riyadh, the Yemeni government and separatists agreed to end their conflict in the southern part of the country.

On Wednesday, Saudi Arabia's King Salman said that agreement could lead to broader peace talks aimed at ending the conflict in Yemen.

The Saudis intervened in Yemen in 2015 at the head of a military coalition against Iranian-backed Huthi rebels, who seized control of the capital Sanaa.

Since then, tens of thousands of people have died in the conflict, most of them civilians, according to humanitarian organizations.

The United Nations, which ranks Yemen as currently the world's worst humanitarian crisis, coordinates relief from more than 250 humanitarian organizations for more than 13 million Yemenis.