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

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

It comes after rebel rocket fire on a residential district of Aden killed 31 civilians on Wednesday and left more than 100 others wounded, according to a medical official.

Rebel shelling on a western district of Aden early on Thursday damaged several homes and left casualties, residents said.

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

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

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

The United Nations on Wednesday declared Yemen a level-three emergency, the highest on its scale, as aid chief Stephen O'Brien held talks to discuss the crisis in the impoverished Arabian Peninsula country.

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