Somali gunmen shot dead a peace activist and kidnapped a senior United Nations official, while a roadside bomb killed three policemen in the anarchic Horn of Africa country, witnesses said on Sunday.

In Beledweyne, central Somalia, assailants assassinated the regional head of respected local non-governmental organisation Centre for Research and Dialogue.

"Men armed with pistols killed Mohamed Hassan Kulmiye in front of a cafeteria," said resident Ismail Farah.

"They shot several bullets in the head. He died on the spot. The men ran away and we do not know who they were."

Suspicion for assassinations and kidnappings generally falls on clan militia and Islamist insurgents who are fighting the Somali government and their Ethiopian military allies.

On Saturday, gunmen also kidnapped the local head of the U.N. refugee agency in Mogadishu in the latest in a spate of abductions of aid workers.

Ten assailants raided the home of Hassan Mohamed Ali, a Somali who runs the agency's operations around Mogadishu, in Elasha, 11 miles south of the capital, residents said.

"They broke into his house after exchanging gunfire with his guards and took him with them," resident Farah Abdi told Reuters. "We see stains of blood in front of his house but we do not know who the kidnappers were and where he is held now."

Kidnapping is lucrative business in Somalia and hostages are generally treated well in anticipation of a large ransom.

Gunmen are still holding four foreign aid workers - two Italians, a Kenyan, a Briton - and three Somalis abducted in April and May.

"We demand the immediate and unconditional release of Hassan Mohamed Ali," said U.N. High Commissioner for Refugees Antonio Guterres, who returned on Saturday from a three-day mission to Kenya that focused on Somalia's dramatic humanitarian situation.

"He and other Somali staff are absolutely crucial in the provision of life-saving humanitarian aid for tens of thousands of innocent civilian victims of the ongoing conflict in their country."

"UNCONSCIONABLE"

Guterres, who visited refugees in the huge Dadaab camp near the Kenya-Somalia border, described the targeting of unarmed, civilian aid workers as "unconscionable" and said the abduction was another blow to humanitarian efforts to help the estimated 1 million uprooted people inside Somalia.

Mired in anarchy and awash with weapons since the 1991 overthrow of dictator Mohamed Siad Barre, Somalia is off-limits for most foreign aid workers, and local staff face extreme risk.

The United Nations withdrew international staff in April after a series of abductions of expatriate aid workers in north-east Somalia, or Puntland, and in south/central Somalia.

Since early 2007, insurgents have been carrying out near-daily, Iraq-style attacks on Somali and Ethiopian security forces. In the latest, a roadside bomb hit a police car in Mogadishu on Sunday, killing three and wounding four others.

Also in Mogadishu, suspected insurgents hurled a hand-grenade into a video hall on Saturday night, killing one person and wounding two more. During their six-month rule of the capital in 2006, the insurgents cracked down on public viewing of films.

An Islamist spokesman claimed the movement's fighters had killed "several" Ethiopian soldiers in an attack in Garasiani town, also in central Somalia, on Sunday. One insurgent died in the clash, added Sheikh Abdirahim Isse Adow.

Residents spoke of five corpses lying on the ground after the fight.

Somalia's prime minister, Nur Hassan Hussein, told a news conference on Sunday that Ethiopian troops - the main target of the Islamists' fury - should withdraw within 120 days as per a U.N.-brokered peace deal the government signed with some opposition figures in Djibouti earlier this month.

Hardliners rejected the agreement, saying there could be no talks until thousands of Ethiopian troops in Somalia left. And the pact has had no impact on fighting on the ground.

"Despite the skirmishes I hope the remaining opposition leaders will join the peace process," prime minister Hussein told reporters.