HARGEISA (Reuters) - A bomb hidden near a mosque killed four policemen in Somalia's northern breakaway enclave of Somaliland on Monday in the latest attack on security forces in the region, police sources said.

Somaliland is proud of its relative stability compared with southern regions of the failed Horn of Africa state, where hardline al Shabaab rebels control large amounts of territory and are fighting a weak Western-backed government.

Washington accuses al Shabaab of being al Qaeda's proxy in Somalia, and security experts say the group wants to extend its influence north -- aiming to destabilise Somaliland and the neighbouring pro-government, semi-autonomous region of Puntland.

No group immediately claimed responsibility for Monday's blast, which came just days after unidentified attackers hurled hand grenades and fired at the main police station in Las Anod near the Puntland border on January 12, wounding three officers.

The police sources said the device had been hidden among milk cans left near Las Anod's Grand Mosque, and it detonated as they inspected the cans. Two policemen were severely wounded.

In two separate violent incidents in the area late on Sunday, gunmen ambushed and killed a senior police operations officer as he returned home from evening prayers, and a woman was killed by an explosion near Las Anod's hospital.

Earlier this month, Somaliland's security forces said they had foiled an attack on a Hargeisa mosque where the imam had spoken out against suicide bombings carried out by al Shabaab insurgents in southern Somalia.

Al Shabaab hit Somaliland and Puntland with synchronised suicide blasts that killed at least 24 people in October 2008. A court in Hargeisa has sentenced five men to death in absentia for those bombings, and said they were on the run in other parts of Somalia. Somaliland has long sought international recognition as sovereign state. It declared itself independent in 1991.

Analysts worry a simmering political row between the president of Somaliland and opposition parties over delayed elections could trigger a re-arming among clan militias, further violence and more turmoil for al Shabaab to exploit.