Kenyan security forces must end the abuse of residents in crackdowns and reprisal raids in the hunt for suspected backers of Somalia's Al-Qaida-linked Shebab militants, Human Rights Watch warned Thursday.

Since Nairobi invaded southern Somalia last October to attack Shebab bases, a string of attacks have rocked the volatile northeastern region, including grenade blasts targeting both the security forces and civilians.

A Kenyan police officer was killed and nine others wounded Wednesday when suspected Islamists hurled a grenade at them during a house raid in a clash that left two assailants dead, police said.

The violence was the latest in a string of clashes and police raids on Kenya's coast, a popular tourist region where Islamists and a separatist movement have increasingly battled with security forces.

Two Kenyan police officers patrolling a city near the border with Somalia were shot dead Sunday by gunmen suspected of sympathizing with Al-Qaida linked Shebab militants, police said.

"The two officers were gunned down while on patrol," a police officer told AFP on condition of anonymity. "Their rifles have been taken away. We highly suspect these are Al-Shebab sympathizers."

The Kenyan army said Friday that it has taken the southern Somali port city of Kismayo, the last stronghold of the al-Qaida linked Shebab, a claim dismissed by residents and the insurgents.

"Kismayo has fallen and with minimum resistance," Kenyan army spokesman Cyrus Oguna told Agence France Presse, even as several residents said that fighting was ongoing and the Shebab still controlled the city centre.

A Kenyan soldier with the African Union force in Somalia killed six people, all thought to be civilians, when he fired on them, a spokesman for the Kenyan Defense Forces said Monday.

"Several people approached a KDF defensive position where a KDF soldier opened fire, killing six people," the army said in a statement.

Fresh violence broke out in Kenya's troubled Tana River region, with at least 20 houses torched in a bitter vendetta between rival groups, the Red Cross said Monday.

"At least 20 houses have been burnt to the ground," Red Cross regional chief Mwanaisha Hami told Agence France Presse, naming the villages as Bura Kipini and Ozi. "The villages were abandoned, no casualties have been recorded."

Villagers targeted in a recent wave of tit-for-tat killings in southeast Kenya say trained militia, including men from outside the region, are behind the raids.

And they suspect politicians may have brought them in.

Tribesmen launched new attacks in Kenya's southeastern Tana River area early Tuesday, torching houses and killing at least three police officers, police sources said.

Officers said armed groups were burning houses and shooting indiscriminately in the Semikaro area in the region's latest round of ethnic violence.

Tribesmen attacked a village in southeastern Kenya Monday, torching homes and sparking clashes that killed 38, in the latest round of tit-for-tat ethnic violence to plague the area, officials said.

The vendetta between the Pokomo farming community and their Orma pastoralist neighbors already left 52 dead last month in Kenya's worst tribal killings in years.

Kenyan gunmen killed at least 12 people in a revenge attack following a massacre last month in which 52 people were killed in the remote southeastern Tana region, Red Cross officials said Friday.

"Eight men, two women and a child died in the attacks," Kenya Red Cross spokesperson Nelly Muluka said, adding that one person later died "from gunshot injuries on his way to hospital".