ADEN - Heavy Saudi-led air strikes and ground combat between armed factions battered southern Yemen on Saturday, killing around 20 Iran-allied Houthi fighters and two rival militiamen, residents said.

Bolstered by more than two weeks of air raids led by Saudi Arabia, local armed groups have been resisting the southward advance of the northern-based Houthis.

Residents said southern fighters ambushed a convoy of Houthis and allied forces loyal to ex-president Ali Abdullah Saleh in a tribal area about 100 km (60 miles) north of their base in Aden, killing 15 of the northerners. Inside the major port city, clashes erupted between Houthi forces and local militiamen firing rocket-propelled grenades and machineguns. Five Houthis and two local militiamen died, residents said.

In the city of Ataq east of Aden, residents reported some 10 air strikes on a military base housing pro-Saleh forces, blowing up ammunition dumps and sending huge fireballs into the air.

Warships believed to be from the Arab coalition shelled Houthi positions and a mountaintop military base run by soldiers loyal to Saleh near the city’s airport.

Saudi Arabia, the world No. 1 oil exporter, is worried the bloodshed could spill over its border with Yemen, and over the influence of Iran, which has denied Saudi allegations that it has provided direct military support to the Houthis.

The Arab Spring uprisings of 2011 failed to produce stable democracies and instead gave rise to sectarian conflicts and the militant groups seeking to seize power. Yemen’s conflict is one of the most complex. Saleh remains influential in the military, despite ceding power in 2012 after mass protests against his rule, and this has complicated efforts to stabilise the poorest Arab state.

While the Houthis deny getting help from Iran and say their armed campaign is designed to stamp out corruption and al Qaeda militants, Saudi Arabia and its allies describe them as an Iranian-backed security threat.

The United Nations says the conflict, in which the Houthis seized the capital Sanaa in northern Yemen in September, has killed 600 people, wounded 2,200 and displaced 100,000 others.

Combat rages across a tangled front covering hundreds of miles of coastline, mountains and deserts. Almost three weeks of heavy street fighting and shelling in Aden has reduced historic pastel buildings beside the commercial port to charred husks.

Food, water and electricity are scarce in Aden. Fighting has cut off key roads linking it to the outside.

“Once this oven stops working, where will our bread come from? What will we eat? We ask God to provide,” said resident Fadhl Mohammed outside a bakery in the Mansoura district.

Meanwhile, local militiamen in the Yemeni city of Aden said they captured two Iranian military officers advising Houthi rebels during fighting on Friday evening.

Tehran has denied providing military support for Houthi fighters, whose advances have drawn air strikes by a regional coalition led by Saudi Arabia, the Islamic Republic’s main rival for influence in the Gulf.

If confirmed, the presence of two Iranian officers, who the local militiamen said were from an elite unit of Iran’s Revolutionary Guards, would further worsen relations between Tehran and Riyadh who are vying for dominance in the region.

Three sources in the southern port city’s anti-Houthi militias said the Iranians, identified as a colonel and a captain, were seized in two separate districts that have been rocked by heavy gun battles.

Meanwhile, a Red Cross plane loaded with medical aid landed in Sanaa on Saturday, the second successive day of such deliveries to the capital of conflict-hit Yemen, a spokeswoman said.

The aircraft was carrying supplies the International Committee of the Red Cross says are urgently needed to treat the wounded after intense fighting between ebels and loyalists of President Abedrabbo Mansour Hadi.

“The new cargo is 35.6 tonnes, of which 32 tonnes is medical aid and the rest water purifying equipment, electric power generators and tents,” said ICRC spokeswoman Marie Claire Feghali.

The ICRC and UN each sent planes to Sanaa on Friday carrying 16 tonnes of medicine and equipment, the first aid supplies to reach the capital since a Saudi-led coalition launched air strikes against the rebels late last month. The United Nations has since called for a daily “humanitarian pause” of a few hours required to deliver aid shipments to the conflict-ravaged country.

On Wednesday two humanitarian aid boats arrived in the main southern city of Aden carrying supplies and personnel destined for people trapped by and wounded in ongoing battles.

The Iran-backed Huthi rebels have seized swathes of territory in Yemen since they entered Sanaa last September, forcing the government to flee.

Yemen has slid deeper into turmoil since the Saudi-led air campaign began on March 26 to push back the rebels’ advance after they forced President Hadi to flee the Arabian Peninsula country.