U.N. Secretary-General Ban Ki-moon called on Thursday for an immediate halt to the war in Yemen, the first time he has made such an appeal since Saudi-led air strikes against Iranian-backed Houthi rebels began three weeks ago.

"I am calling for an immediate ceasefire in Yemen by all the parties," Ban said in a speech to the National Press Club in Washington. "The Saudis have assured me that they understand that there must be a political process. I call on all Yemenis to participate in good faith," he added.

The secretary-general's comment came a day after the UN envoy to Yemen, Jamal Benomar, announced his resignation. The veteran Moroccan diplomat was the one who helped broker a 2011 transition plan in Yemen that later unraveled.

Ban Ki-moon said that even before the latest escalation, two out of three Yemenis relied on humanitarian assistance, noting that the need for food and security had been higher than the poorest stretches of Africa.

"Recent fighting has only multiplied the sufferings and security. Hundreds are dead, humanitarian supplies are being blocked and UNICEF has reported that an astounding one third of fighters are children," he added, using an abbreviation for the United Nations Children's Fund.

Warplanes from Saudi Arabia and Sunni Arab allies have been striking Shia Houthi fighters battling to oust U.S.-allied Yemeni President Abd-Rabbu Mansour Hadi, in an attempt by the regional heavyweight to check Iranian influence in its backyard.

The Sunni Gulf states resorted to use of force to push the Houthis back to a UN-backed political transition, which was interrupted by the group's seizure of capital Sana'a last September.