* U.N. rights boss says both sides attacking civilians

* Rebels said to execute civilians, plant mines & bombs

* Rebel courts issue death penalty by stoning, decapitation

(Recasts, adds details on violations, UN humanitarian concerns)

By Stephanie Nebehay

GENEVA, July 10 (Reuters) - The United Nations human rights chief said on Friday both sides in Somalia's conflict are committing attacks and using torture against civilians, which may amount to war crimes.

Islamist insurgents are executing civilians, planting mines and bombs in residential areas and using torture while their tribunals hand down death sentences by stoning and decapitation, U.N. High Commissioner for Human Rights Navi Pillay said.

Pro-government forces are also said to have committed grave violations, including firing mortars on residential areas and using torture, she said in a statement.

Civilians, especially women and children, are bearing the brunt of the latest violence in the Horn of Africa country, Pillay said as government troops sought to drive insurgents from their bases in the capital Mogadishu. [ID:nLA469466]

"Witnesses have told U.N. investigators that the so-called al Shabaab groups fighting to topple the transitional government have carried out extrajudicial executions, planted mines, bombs and other explosive devices in civilian areas and used civilians as human shields," Pillay said.

"Fighters from both sides are reported to have used torture and fired mortars indiscriminately into areas populated or frequented by civilians," she said. "Some of these acts might amount to war crimes".

Al Qaeda-linked fighters in al Shabaab control much of southern and central Somalia and all but a few blocks of the capital. Western powers fear that if the Somali government is overthrown, the country will become a safe haven for al Qaeda training camps and that militants will destabilise the region.

'DEVASTATING CONSEQUENCES'

Concerns are also increasing about living conditions inside the country, where aid agencies are struggling to reach needy people including more than 200,000 who have fled the capital.

"During the past week, heavy fighting in Mogadishu continued with devastating consequences for the civilian population and limited access for humanitarian agencies," the U.N. humanitarian agency OCHA said in its latest weekly bulletin issued on Friday.

It quoted the local Elman Peace and Human Rights Organisation as saying at least 350 civilians had been killed and more than 1,500 wounded since the offensive led by al Shabaab and Hisbul Islam militia began in May.

In the Somali town of Baidoa on Friday, witnesses described how hardline Islamist rebels beheaded seven people for being "Christians" and "spies" in the latest implementation of strict sharia law by the al Qaeda-linked al Shabaab group.

Pillay, a former U.N. war crimes prosecutor, said that human rights activists, aid workers and reporters in Somalia are at high risk. Six journalists have been killed in Mogadishu this year, including four apparently assassinated, she said.

"Once order has been restored -- and one day order will be restored -- those responsible for human rights violations and abuses should, and I hope will, be brought to justice," said Pillay, who is from South Africa.

Her spokesman Rupert Colville, asked whether a case could be brought to the International Criminal Court, noted that the Hague-based tribunal had indicted leaders of Uganda's Lord's Resistance Army and Democratic Republic of Congo warlords. (Additional reporting by Tsegaye Tadesse in Addis Ababa) (Editing by Diana Abdallah)

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