Four people have been killed after Islamist insurgents launched a mortar attack on the presidential palace in the Somali capital, Mogadishu.

President Abdullahi Yusuf was there at the time but no mortars landed inside the compound, a security officer said.

Mogadishu has seen rising violence since Ethiopian-backed government troops ousted Islamists last December.

Hundreds have since died in clashes between Islamist-backed insurgents and government-backed Ethiopian troops.

The BBC's Mohammed Olad Hassan says at least three mortars were fired at the palace - know as Villa Somalia.

One of the mortars landed on a home, killing three children and their mother and injuring several other civilians.

Mortars were also fired at the main football stadium in south Mogadishu, which serves as the base for Ethiopian forces, our correspondent says.

"We had about five heavy explosions inside the stadium but we do not have any idea of the Ethiopian casualties," Abdi Qdar, who lives close to the stadium, told BBC News.

Witnesses said the Ethiopian troops responded by firing mortars, which landed on homes surrounding the base, injuring civilians.

UN humanitarian chief John Holmes says the situation in Mogadishu is worsening, and that recent fighting has hampered the delivery of humanitarian aid.

The UN refugee agency says some 400,000 people have fled the fighting in the capital in the past four months.