Anti-Houthi fighters of the Southern Popular Resistance stand near a tank in Yemen's southern port city of Aden May 16, 2015. [Photo/Agencies]

RIYADH - A Saudi Arabian-led coalition resumed air strikes against Yemen's Houthi militia in Aden overnight, hours after the expiry of a truce meant to facilitate badly needed humanitarian aid, a eyewitness said.

The witness said explosions could be heard near the southern city's airport and the districts of Khor Maksar and Crater shortly the five-day ceasefire expired on Sunday at 2000 GMT. No further details were immediately available.

Late on Sunday a spokesman for the army, much of which is allied to the Houthis, welcomed a request by the UN envoy to Yemen to extend the truce to allow more aid to be delivered to the war-damaged Arabian Peninsula country.

"We welcome the call by the UN envoy to Yemen ... regarding the extension of the truce and the need to deliver humanitarian aid to citizens," Yemen's Houthi-controlled state news agency SABA quoted Brigadier General Sharaf Luqman as saying.

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 Yemen's political turmoil.

Since Tuesday Saudi-led forces and Yemen's Houthi militias had largely observed a ceasefire meant to allow 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 had continued, however, with at least 15 killed overnight Saturday-Sunday 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.