Somali gunmen have shot and killed a truck driver for the United Nations World Food Programme (WFP), the fourth this year, the agency said Wednesday.

Ahmed Saalim was part of a convoy of trucks carrying 602 metric tons of food from Mogadishu to Bay and Bakool regions, the WFP said.

The agency called on all parties to ensure the safe passage of humanitarian convoys amid a worsening security situation.

'WFP food is reaching many people, but our drivers are daily risking their lives to deliver it,' said Peter Goossens, WFP Somalia Country Director.

A combination of conflict, drought and rising food and fuel prices means that millions of Somalis are dependent on humanitarian aid.

The WFP said it must double food assistance to feed around 2.4 million Somalis for the rest of the year.

However, the task is being complicated by rampant piracy off the coast and by the attacks on humanitarian workers.

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.

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 Ayro's death.

Gunmen killed the head of the UN Development Programme on Sunday and the head of the UN refugee agency UNHCR's Mogadishu programme is still being held.

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.