SANAA, Yemen (AP) — Yemen's Shiite rebels along with security forces loyal to the former president began a fresh offensive Monday against the southern city of Aden, shelling the city with artillery and battling local militias as Saudi airstrikes continue to target rebel positions, according to security officials.

Aden, the economic center of this impoverished nation, was declared Yemen's provisional capital by embattled President Abed Rabbo Mansour Hadi before he fled the country last week. The rebels, known as Houthis, overran the capital, Sanaa, in September — eventually placing Hadi under house arrest and forcing him to flee the city. The Houthis are allied with former president Ali Abdullah Saleh, forced out as part of the country's 2011 Arab spring uprising, and have been joined by army and security forces loyal to Saleh.

According to officials, the combined force of Houthis and Saleh loyalists is positioned about 30 kilometers (19 miles) east of Aden, near the southern city of Zinjibar. The rebels have used artillery barrages to target pro-Hadi militias known as the Popular Committees.

There was no immediate report on causalities.

Meanwhile in Sanaa, a series of airstrikes shook the city overnight and early Monday morning as the Saudi-led coalition campaign against the Houthis continued for a fifth day. The strikes have targeted militants, jets, air defense systems and Scud missile launch pads that could threaten Saudi Arabia.

Several days of airstrikes have bred a climate of anxiety and uncertainty in Sanaa. Schools were canceled, residents sought shelter indoors, and hundreds fled to the safety of nearby villages.

As the strikes were taking place, Arab leaders meeting in Egypt for the two-day Arab League summit unveiled plans to form a joint Arab military intervention force — setting the stage for a potentially dangerous clash between U.S.-allied Arab states and Iran over influence in the region. Critics of the Houthis charge that they are a proxy for Iran's regional ambitions, an accusation both the rebels and Tehran deny.

So far the strikes have targeted eight out of Yemen's 21 provinces. Since the air campaign began, the Houthis have arrested about 140 foreign nationals on suspicion that they are providing the Saudis with intelligence on the locations of army barracks, radars and air defense positions, according to the Yemeni Interior Ministry.