ADEN: Rebel fire on a residential district of Yemen's second city Aden killed more than 30 civilians Wednesday, as the UN declared its highest level humanitarian emergency in the war-torn country.

In the central city of Taez, meanwhile, pro-government forces launched a manhunt for 1,200 escaped prisoners.

Both Aden and Taez have seen heavy fighting as loyalists of exiled President Abedrabbo Mansour Hadi battle to repel Shia Houthi rebels with the support of a Saudi-led air war launched in March.

Aden was Hadi's last refuge before he fled into exile in neighbouring Saudi Arabia in March and his supporters have been battling to defend it against the rebels and renegade troops.

The Houthis and their allies pounded the loyalist-held Al-Mansura district of Aden with 15 Katyusha rockets, loyalist forces spokesman Ali al-Ahmadi said.

The rocket fire began before dawn when the streets were busy ahead of the daytime fast observed by Muslims during the holy month of Ramadan.

A fresh salvo of rockets later in the morning hit mourners burying some of the dead from the earlier fire, the spokesman and witnesses said.

The city's health chief Al-Khader Laswar said at least 31 people were killed, including three women and two children, while more than 100 others were wounded.

Civilians were seen carrying bloodied bodies and calling for help as they piled them into vehicles and drove them to hospitals.

Highest-level emergency

Overnight, rebel positions in the nearby neighbourhoods of Dar Saad and Khor Maksar had been hit by a series of Saudi-led air strikes, said residents.

A coalition strike in neighbouring Lahj province killed 13 rebels, an official said.

On Tuesday, Human Rights Watch said strikes on the rebel stronghold of Saada in Yemen's northern mountains had destroyed houses, markets and a school, in what could amount to war crimes.

Take a look: Saudi strikes hit homes, markets in Yemen, claims HRW.

Across the border in the Saudi city of Jeddah, Yemen's exiled Prime Minister Khaled Bahah accused the rebels of committing a “war crime” in Aden by attacking residential areas, laying siege on the city, and forcing aid vessels to turn back.

On the humanitarian front, the United Nations declared Yemen a level-3 emergency, the highest on its scale.

UN aid chief Stephen O'Brien met with heads of agencies to discuss the crisis in the impoverished Arabian peninsula country.

“All agencies agreed to declare the level three for a period of six months,” said UN spokesman Farhan Haq.

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

Under the emergency plan, the United Nations will try to reach 11.7 million people most in need.

“The health system is facing imminent collapse with the closure of at least 160 health facilities due to insecurity, lack of fuel and supplies,” Haq said.

Qaeda members escape

In Taez, Yemen's third biggest city, loyalist forces were searching for 1,200 inmates, including Al Qaeda members, who made a mass breakout as the prison was captured from rebel forces.

A loyalist source accused the rebels of deliberately throwing open the gates in an apparent attempt to cover their withdrawal.

“Between five and eight Al Qaeda members were among the prisoners,” a military source said.

There have been repeated jailbreaks in Yemen since the Houthis launched an offensive last summer, overrunning the capital and then much of the rest of the country.

Al Qaeda's Yemen arm took advantage of the rebellion to seize the southeastern port city of Mukalla in April where it freed more than 300 inmates, including one of its leaders.

Washington regards Al Qaeda in the Arabian Peninsula as the network's most dangerous branch and has kept up a drone war against its leaders.

But the Islamic State group too has exploited the conflict to enter the fray, carrying out a string of deadly attacks against Shia targets since March.

An attack claimed by the Sunni extremists on Houthi leaders in the rebel-held capital Sanaa killed at least 28 people on Monday.

The United Nations has called repeatedly for a humanitarian ceasefire to allow the delivery of desperately needed relief supplies.

UN envoy Ismail Ould Cheikh Ahmed renewed the appeal in talks with Hadi in the Saudi capital late on Tuesday, a government official said.