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 Wednesday and left more than 100 others wounded, according to a medical official. Rebel shelling on a western district of Aden early 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 Abed Rabbou 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.