Mogadishu - Unknown gunmen shot dead the head of the United Nations' Development Programme as he left a mosque near his home in the Somali capital Mogadishu, witnesses said Monday.

Osman Ali Ahmed was attacked Sunday evening in Bula Hubey, south Mogadishu. He later died of his injuries in an African Union peacekeepers' hospital.

"As we came out from the mosque after prayers, three men armed with pistols began simultaneously firing at Ahmed's head and body," Isse Dirie, a witness, told Deutsche Presse-Agentur dpa.

One of Ahmed's companions was also injured in the attack.

Aid workers have been increasingly targeted for attacks and abduction since the man believed to be al-Qaeda's top operative in Somalia, Aden Hashi Ayro, was killed on May 1 in a US airstrike.

The head of the UN refugee agency UNHCR's Mogadishu programme was recently kidnapped and is still being held.

Several other aid workers have been held captive for months, and the World Food Programme has seen three of its drivers slain this year.

Ayro was the leader of Islamic militant group al-Shabaab, the armed wing of the Union of Islamic Courts (UIC). Al-Shabaab said it would target foreign troops and workers to avenge the deadly airstrike.

Militants have been waging a guerrilla war against government troops since the UIC was ousted from power at the beginning of 2007 with Ethiopian assistance.

The interim government has been unable to achieve stability in the Horn of Africa country, which has been plagued by chaos and civil war since dictator Mohamed Siad Barre was toppled in 1991.

A peace deal was agreed between moderate Islamists and the government in early June, but al-Shabaab has not signed the agreement and has vowed to keep fighting until Ethiopian troops leave Somalia. (dpa)