(Adds details, Mogadishu)

By Abdiqani Hassan

BOSASSO, Somalia, Feb 5 (Reuters) - An explosion killed at least 20 people and wounded a hundred more on Tuesday in a northern Somali port where Somali and Ethiopian immigrants begin the dangerous crossing to Yemen, a regional official said.

"A big explosion killed more than 20 people and wounded at least a hundred tonight," Bari regional governor Musa Gelle Yusuf told Reuters.

"More dead bodies are still being brought to Bosasso hospital. We do not know the cause but police are investigating," he said.

Most of the wounded were Ethiopian immigrants, Bosasso hospital nurse Mohamed Ali Hassan said. Ethiopia has for hundreds of years been the chief rival of Somalia.

Thousands of people fleeing poverty and violence in Somalia and neighbouring Ethiopia risk passage through the Horn of Africa nation to make the trip to Yemen in hope of better lives.

But the journey across the Gulf of Aden is treacherous, and kills hundreds annually when often-overloaded boats capsize or sink in the shark-infested waters.

Bosasso is the main port in the semi-autonomous northern Puntland region, which is relatively peaceful compared with southern Somalia, where the interim Somali federal government is battling an insurgency led by Islamist militants.

In the Somali capital Mogadishu, four people were killed by insurgents in the northern part of the city, witnesses said.

"The insurgents first killed a policewoman and then they chased a policeman who ran when he saw them," resident Quresh Osman said. "The insurgents then opened fire indiscriminately, killing the policeman and two other civilians."

A bloody insurgency pitting the government and its Ethiopian military allies against the remnants of a militant Islamist movement has raged in the capital. The movement was run out of the city a year ago.

At least 6,500 people have been killed and more than 600,000 forced to flee near-daily violence.

In Baladwayne, near the Ethiopian border, four gunmen shot dead the local district attorney, a regional official and a resident said.

"It is true that the district attorney of Baladwayne, Sheikh Ibrahim Gonle, was killed this evening. We do not know yet who killed him," Hiraan regional chairman Yusuf Ahmed Hagar told Reuters by telephone.

A witness, Omar Mohamud, said four men shot the attorney and wounded one of his bodyguards.

Hiraan region has seen a few episodes of violence by pro-Islamist gunmen in the past few months, but residents say it has mostly been sparked by dissatisfaction with the regional administration's levying of high taxes. (Additional reporting by Abdiwahid Sheikh, Writing by Bryson Hull; Editing by Elizabeth Piper)

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