Dust rises from the site of a Saudi-led air strike in Yemen's southwestern city of Taiz yesterday. Reuters

 AFP/Aden

Fighting raged in Yemen's battleground southern port of Aden Thursday, a day after the United Nations declared its highest level of humanitarian emergency in the war-torn country.

The clashes killed seven rebels and five pro-government fighters, a military official said.

The fighting comes a day after rebel rocket fire hit a residential district of Aden, killing 31 civilians and wounding more than 100.

Early Thursday, rebel shelling of a western district damaged several homes and left casualties, residents said.

Meanwhile, a port near the Aden oil refinery came under rebel artillery fire for a fifth consecutive day, as a fire continued in the area, said Aden Refinery Co spokesman Naser al-Shayef. 

In adjacent Lahj province and nearby Shabwa, Saudi-led coalition warplanes carried out several strikes on rebel positions overnight, residents said.

The coalition has been bombing the Iran-backed Houthi rebels and their allies since March 26 in support of Yemen's President Abedrabbo Mansour Hadi, who fled to Saudi Arabia.

Sanaa, meanwhile, was rocked by blasts Thursday as coalition jets carried out several raids on rebel-held weapons depots and other positions across and around the capital. 

Amnesty International warned of "the high price civilians continue to pay amid the...  air strikes all over the country" and accused the coalition of failing to "abide by the requirements of international humanitarian law."

That requires belligerents to "take all possible steps to prevent or minimise civilian casualties," said Donatella Rovera, senior crisis response advisor at Amnesty.

But there is "no indication" the coalition has done "anything to prevent and redress such violations," said Rovera, who is currently in Yemen.

More than 2,600 people have been killed in the impoverished Arabian Peninsula country since March, according to UN figures.

On Wednesday, the United Nations declared Yemen a level-three emergency, the highest on its scale, as aid chief Stephen O'Brien held talks to discuss the crisis there.

More than 21.1 million people -- over 80 percent of Yemen's population -- are in need of aid, with 13 million facing food shortages.