Fire erupted at Aden's oil refinery Saturday when rebels shelled the nearby port to prevent a Qatari ship carrying aid for Yemen's devastated second city from docking, officials told AFP.
"The rebels fired artillery rounds at the area and one hit an oil tank at the refinery, sparking a fire," said an official at the Aden Refinery Company.
An AFP reporter saw flames raging as pillars of smoke billowed into the skies over the southern city.
A government official told AFP the rebels had targeted a Qatari vessel carrying food supplies from Djibouti, a hub for Yemen-bound humanitarian aid, and that the attack forced it to turn back.
Both the port and the refinery in Aden's Buraiqa district are controlled by pro-government fighters, and the area has seen fierce clashes between rival forces.
The refinery has not been receiving any oil via the port, but it still has 1.2 million tonnes of crude in storage and also gas tanks.
Shiite Huthi rebels who overran the capital Sanaa in September have since expanded their control to several other regions.
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In March they advanced on Aden, where President Abedrabbo Mansour Hadi had taken refuge before fleeing to Saudi Arabia.
A Saudi-led coalition began a bombing campaign against the rebels and in support of Hadi in late March.
Coalition warplanes carried out "at least 15 air raids" across several parts of Aden on Saturday, a pro-Hadi military official said.
Meanwhile, five pro-government fighters and two civilians were killed in clashes rocking Aden on Saturday, medics and militia sources loyal to Hadi said.
More than 2,600 people have been killed in the impoverished Arabian Peninsula country since March, according to UN figures. Almost 80 percent of the population -- 20 million people -- need urgent humanitarian aid.
The situation is particularly serious in Aden, where residents complain of food and water shortages and health officials warn of disease spreading.
Earlier this month, a UN-chartered ship carrying humanitarian supplies bound for Yemen was targeted by shelling as it approached Aden.
In another southern province, Shabwa, tribesmen killed nine rebels in an attack on a troop carrier on Saturday, a military source close to the Huthis told AFP.
Rebel forces comprise both Huthis and renegade troops loyal to former president Ali Abdullah Saleh.