The conflict has disrupted imports in Yemen, where about 20 million people or 80 percent of the population are estimated to be going hungry, a statement by the United Nations and the Yemen International NGO Forum said.

A shortage of fuel has crippled hospitals and food supplies in the past few weeks, and the U.N.'s World Food Programme has said its monthly fuel needs have leapt from 40,000 liters a month to 1 million liters.

"Millions of lives are at risk, in particular children, and soon we will not be able to respond," Edward Santiago, country director for Save the Children, said in the statement.

POSSIBLE TRUCE

The statement also dismissed an announcement by the Saudi-led Arab alliance about a possible truce in some areas to allow for humanitarian supplies, saying it was not enough.

"The recent announcement of a potential humanitarian pause to military operations will not alleviate the humanitarian impacts of the current conflict," the statement said, calling for a permanent end to hostilities.

The United Nations said on Tuesday at least 646 civilians has been killed since coalition air strikes began on March 26, including 131 children, with over 1,364 civilians wounded.

Iranian-allied Houthi fighters, backed by forces loyal to former President Ali Abdullah Saleh, have seized control of large parts of Yemen in what they say is a campaign against al Qaeda militants in the country.

The Saudi-led coalition, including nine Arab states provided with logistical support by the United States, France and Britain, seeks to restore the government of President Abd-Rabbu Mansour Hadi, now in exile in Riyadh.

On Tuesday, mortars and Katuysha rockets fired by Houthi fighters in Yemen struck the Saudi city of Najran, the coalition's spokesman said.

Saudi-owned al-Arabiya television reported on Wednesday that a man and a woman were also killed in the city of Jizan when shells fired by Houthis fell on a family house. A girl was also severely wounded in the attack, the satellite channel said.

SAADA IS HOUTHI STRONGHOLD

Saudi Brigadier General Ahmed Asseri said the projectiles on Tuesday struck a girls' school and a hospital in Najran, which is only 3 km (2 miles) from Yemen's border, prompting authorities to close down all schools in the area.

Civil defense authorities in Najran said three people died and 37 were wounded in the attacks, which hit buildings and cars and blasted holes in the pavement, according to Saudi media.

Saudi-led warplanes responded overnight with more than 30 air strikes on the northwestern Yemeni provinces of Saada and Hajja near the Saudi border, local officials and residents said.

Saada is a stronghold of the Iranian-allied Houthi movement.

Houthi sources said 43 civilians were killed and at least 100 wounded as a result of the strikes, which lasted until dawn on Wednesday. The figure could not be independently verified.

Local sources also said there was heavy artillery shelling coming from the Saudi border.

Reuters