BAGHDAD, (Reuters) – Iraq’s prime minister sent an envoy to the southern city of Amara on Friday after clashes between Shi’ite militias and police in areas U.S. and British forces handed over to Iraqi control months ago.

Violence between Shi’ite militias and Iraqi security forces, fuelled by tribal divisions, have killed 15 people and wounded 91 since Thursday, a security source in Amara said.

National Security Minister Shirwan al-Waeli told Reuters by telephone on his way to Amara that the situation was “serious”, but he denied reports the city was totally under control of the Mehdi Army, a Shi’ite militia loyal to cleric Moqtada al-Sadr.

A spokesman for the British military, which handed over responsibility for the city to Iraqi forces around two months ago, said the situation remained volatile.

The city of Balad, scene of reprisal killings that have left at least 60 dead in the past week, was also on alert again after mortars killed nine people on Thursday, prompting Shi’ite militias to attack two nearby Sunni villages, police said.

Balad and Amara are both cities handed over to Iraqi control by U.S. or British forces in recent months.

Baghdad was under a regular Friday curfew the day after Washington said the results of its two-month-old campaign to curb violence through mass reinforcements had been a disappointment and it was rethinking the plan.

The setbacks in the battle for control of Baghdad, which U.S. officials say will decide Iraq’s future, and rising U.S. casualties have piled pressure on U.S. President George W. Bush before November mid-term elections.

The Shi’ite-led Iraqi government is struggling to exert its authority over Shi’ite militias blamed for reprisal killings, and to build a viable police force capable of retaining its independence in areas with deep tribal or sectarian loyalties.

Prime Minister Nuri al-Maliki, who this week met powerful Shi’ite clerics Sadr and Grand Ayatollah Ali al-Sistani to enlist their support in curbing sectarian violence, ordered Waeli to go to Amara on Friday to try to quell the turmoil.

Waeli said the trouble started with the assassination of the head of police intelligence. “Amara is a tribal town so it turned into a crisis,” Waeli said, adding that police neutrality was a problem because some officers were loyal to their tribes.

“Our priority is to stop the fight. All the religious leaders have given their instructions to calm down the situation,” Waeli said. “The government is very serious in not allowing the forces to be loyal to any political group.”

An envoy from Sadr also arrived in Amara on Friday, the security source there said.

The source said Thursday’s militia attacks started after the disappearance of the brother of a senior Mehdi Army leader. Suspecting he was detained by police, militias attacked police stations with rocket-propelled grenades and rifle fire.

British Major Charlie Burbridge in Basra said the Iraqi Army had deployed two companies (around 230 troops) to help defend the police stations on Thursday. British forces had provided air surveillance and were ready to offer more assistance if asked.

The security source in Amara said the police seized control on Friday of the Sadr office in Amara. The militias then renewed their attacks on at least three police stations.

Balad, 80 km (50 miles) north of Baghdad, was also in the spotlight again after a week of attacks that left dozens dead.

Police said at least 15 mortars hit the city on Thursday, killing nine people, and shortly afterwards gunmen wearing the black outfits associated with local Shi’ite militias attacked two Sunni villages. They had no information on casualties.

Iraq is gripped by bloodshed between Shi’ite and Sunni Muslims, and Balad, a largely Shi’ite city surrounded by Sunni villages, illustrates the threat of civil war.

U.S. forces, who maintain a large base near Balad, sent troops back into Balad earlier this week.