At least seven civilians have been killed and 36 injured when Houthi rebels and forces loyal to former President Ali Abdullah Saleh targeted residential areas in Yemen's Taiz, medical sources have told Al Jazeera.

Monday also saw more than 50 Houthi rebels and pro-Saleh fighters killed in the southern city during Saudi-led Arab coalition air strikes and a battle with supporters of President Abd-Rabbu Mansour Hadi.

Eight pro-Hadi forces were also killed in the fighting.

The intensifying violence has left Taiz in a desperate situation, with closed hospitals and acute shortages of medicine, food, water and fuel, the Red Cross said this week.

Meanwhile, a suicide bomber killed two pro-Hadi fighters from the southern separatist movement at a checkpoint in Aden on Monday, security and medical officials told the AP news agency.

The bomber, wearing a suicide belt, blew himself up in the Mansoura neighbourhood of the southern port city.

The attack followed Sunday's storming of an Aden supermarket by fighters who fired shots into the air and briefly took hostages, according to security officials and witnesses.

Witnesses said the attackers warned supermarket employees against the mingling of men and women and demanded that female employees cover their faces.

Following a major battle over Aden between Houthi rebels and pro-government forces who pushed the Houthis out of the city in July, al-Qaeda appears to have taken advantage of the local security vacuum as pro-government forces moved on to challenge the Houthis elsewhere in southern Yemen.

Southern separatist Adel Muhasin said al-Qaeda had largely succeeded in penetrating local Salafi groups and now holds several areas, with about 5,000 fighters organised under its command in Aden.

"They have the money and the weapons, so they've succeeded in recruiting hundreds of young people, many of them brainwashed or seeking money," he said.

Source: Al Jazeera and agencies