The U.N. envoy to Yemen on Sunday called on a Saudi-led military coalition and its adversaries to extend a humanitarian truce due to expire at 2000 GMT (4 p.m. ET), to allow for more aid to be delivered to Yemenis.

Mauritanian diplomat Ismail Ould Cheikh Ahmed made his appeal at the opening of a conference of Yemeni parties that convened in the Saudi capital Riyadh to discuss ways of ending political turmoil in the impoverished Arabian Peninsula country.

Since Tuesday Saudi-led forces and Yemen's Houthi militias have largely observed a ceasefire meant to allow for delivery of food, fuel and medical supplies to millions of Yemenis caught in the conflict since the alliance began air strikes on March 26.

Sporadic clashes have continued, however, with at least 15 killed overnight in the cities of Taiz and Dhalea, residents said.

Relief groups say that the five days were hardly enough to allow sufficient supplies to reach the country of 25 million.

Impoverished and strife-torn even before the war, Yemen is now mired in a humanitarian catastrophe, as 300,000 people have been displaced by the conflict and 12 million are short of food.

"I am hopeful (there will be an extension)," Ould Cheikh Ahmed told journalists on the sidelines of the conference, attended by President Abd-Rabbu Mansour Hadi. "All ‎my first contacts indicate that we have a chance, but I am really calling on all parties to extend this for a minimum of five days."

Saudi Arabia has said that extending the truce depended on how the Houthis and their ally, former President Ali Abdullah Saleh, complied with the ceasefire.

The world's top oil exporter has accused the Houthis of violating the truce but said it would exercise self-restraint to allow the delivery of badly needed supplies to Yemenis.

Yemeni Vice President Khaled Bahah, who also heads the government in exile, said his administration was in favor of extending the truce but a decision on that depended on the situation on the ground.

"We need the ceasefire to continue for long, not just for a few days, but it depends on the operation on the ground," Bahah told Reuters in the Saudi capital, Riyadh.

"There is an effort for an extension, but it depends on how it is on the ground. But it's our wish from the government side that we need to extend it," he added.

AID, FIGHTING

Hadi told the conference that the Iran-allied Houthis must quit cities they had captured since they moved on Sanaa and pushed his government aside in September last year, adding that they should not be allowed to "take advantage of the truce to expand more and kill civilians."

The conference was rejected by both the Houthis and fighters loyal to former president Saleh, meaning it will not provide an opportunity for peace talks.

However, some leading figures from Saleh's party, the General People's Congress (GPC), have traveled to Riyadh and pledged loyalty to Hadi's government.

Ould Cheikh Ahmed said the United Nations, which had previously sponsored Yemeni discussions on ways of ending the crisis before the Saudi-led air strikes began, was looking to resuming the talks in Geneva around the end of May.

"We need flexibility, we need open minds... and we need them to come with an open mind and no preconditions," he said.

Aid flights have started from the United Arab Emirates to the capital Sanaa, which is under Houthi control and has faced air strikes but no ground fighting. The United Nations said aid ships had docked at ports in Hodeida and Aden.

On Friday, the World Health Organization (WHO) said more than 20 tonnes of medical supplies had been sent from Dubai to Djibouti for delivery to Yemen. It said it was increasing shipments of medical supplies during the truce.

A shipping source in Yemen said seven ships with fuel, wheat and food had docked in Hodeida and in al-Mukalla on Friday.

On Sunday, a vessel with more than 30,000 food parcels sent by the UAE docked at al-Buraiqa port, near the oil refinery in the southern city of Aden. Aden itself is under control of anti-Houthi forces, according to shipping sources in the city.

The sources said this was the fist aid consignment to reach Aden, which has been devastated by heavy fighting.

In some of the worst clashes since the truce, at least 10 people died in overnight battles in Taiz city between Houthis and local militiamen, residents and medical sources said.

Some fighting also took place in the city of Dhalea on Saturday night and residents reported at least five people killed. Clashes were under way in Shabwa, a province east of Aden, where local fighters were trying to wrest back the provincial capital, Artaq, from the Houthis.

Reuters