ADEN/CAIRO - Over 120 people, mostly civilians, have been killed in heavy fighting across Yemen, rescue workers and Houthi sources said on Wednesday, as aid agencies warned fuel shortages could halt their urgent work.

The heaviest clashes were concentrated in the southern port city of Aden, where at least 40 civilians trying to flee the city by boat died when shells fired by Houthi fighters struck their vessel, rescue workers and witnesses said.

Earlier in the day, Houthi sources said 40 civilians were killed when warplanes from a Saudi-led coalition struck Yemeni provinces near the Saudi border overnight, following the death of three Saudis in the first deadly cross border attack since the Arab alliance began operations in March.

The conflict has disrupted imports to 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 recent weeks, and the UN’s World Food Programme has said its monthly fuel needs have leapt from 40,000 litres a month to 1 million litres.

“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.

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 a permanent end to hostilities was needed.

US Secretary of State John Kerry said on Wednesday Washington was concerned about the dire humanitarian situation in Yemen and pledged $68 million for relief work in the country. He told journalists in Djibouti that he would discuss a possible pause in fighting with Saudi officials later on Wednesday to try to get food, fuel and medicine to civilians “We have urged all sides to comply with humanitarian law to take every precaution to keep civilians out of the line of fire,” he added. In the city of Aden, rescue workers and witnesses said about 40 civilians were killed when a shell fired by the Houthis struck a boat they were on trying to flee the al-Tawahi neighbourhood to safer grounds.

Residents and local fighters said Tawahi had witnessed intense clashes with Houthi fighters based nearby, which had drawn Saudi-led airstrikes. They said at least 30 Houthi gunmen and 10 local fighters died in overnight fighting, including a local commander, Brigadier General Ali Nasser Hadi, who was reportedly killed by a Houthi sniper.

Earlier on Wednesday, Saudi-owned al-Arabiya television said two people were killed and a child severely wounded in the Saudi city of Jizan overnight when Houthis fired shells on a house.

In Yemen, another nine people were killed and 18 were injured in air strikes on a police academy in Dhamar province, south of the capital Sanaa, the Houthi-run Saba news agency said on Wednesday. The United Nations said on Tuesday at least 646 civilians had been killed since coalition air strikes began, including 131 children, with over 1,364 civilians wounded.

The Iranian-allied Houthi fighters, backed by forces loyal to former President Ali Abdullah Saleh, have seized control of key parts of Yemen, including the capital.

Riyadh accuses Tehran of providing weapons to the Yemeni Shi’ite Muslim militia, a charge Iran denies.

“Yemenis don’t need our weapons. Saudis’ actions in Yemen are not justifiable ... Americans are helping them... we only wanted to send them (Yemenis) medicine,” Iranian state television cited Supreme Leader Ali Khamenei as saying.

The Saudi-led coalition, which includes nine Arab states and has logistical support from 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 Katyusha rockets fired by Houthi fighters in Yemen struck the Saudi city of Najran, close to the Yemen border, the coalition’s spokesman said.

Civil defence 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.