NAIROBI (AFP) -  The African Union force in Somalia apologized Friday for the "tragic" killing of seven civilians at a wedding last month, an attack it previously denied, saying three soldiers had been charged.

Witnesses in Somalia's southern port town of Merka told AFP that troops from the 22,000-strong AU force in Somalia (AMISOM) had opened fire after insurgents hurled a grenade at a passing convoy on July 21.

Hours after the attack, AMISOM said it "categorically" denounced the "devious allegation."

Human Rights Watch (HRW) later alleged that Ugandan troops with AMISOM - which fights alongside Somali government forces against the Al-Qaeda affiliate, the Shabab - massacred six men in "cold blood" at a wedding after a bomb attack.

"At one house, where the Moalim Iidey family was celebrating a wedding, the soldiers separated the men from the women and shot the six adult men - four brothers, their father, and an uncle," the HRW report read.

"Four died immediately, one brother hid under a bed after being shot but later died, and the father died during the night after the soldiers allegedly refused to allow the family to take him to the hospital."

On Friday, AU envoy and AMISOM head Maman Sidokou admitted the killings of civilians had taken place.

"We have established that, on that occasion, seven civilians died following an incident involving our troops," Sidokou said in a statement released in Kenya.

"I would like on behalf of the African Union, to offer my sincere apology for these deaths. We regret these deaths."

Three AMISOM soldiers have been "indicted and are awaiting to be arraigned before a military judicial process," he added.

"I have instituted a board of inquiry composed of military, civilian and police officers who are not from the contingent concerned in order to ensure impartiality."

Deployed to Somalia since 2007, AMISOM has helped push back Shabab across much of the country's south, retaking towns and territory the group had held for years.

"Our soldiers are operating in a very complex environment," Sidokou added.

"We will continue to appeal to our host communities to facilitate the work of our troops in the fight against Al-Shabab."