Mogadishu - The African Union mission has denied charges that its troops killed 20 civilians in Somalia.Medical sources and a legislator said on Wednesday that AU troops killed the civilians in the southern port town of Merca.The death toll was initially given as 10 in Tuesday's attack.Eighteen people were hospitalised with gunshot wounds, medical official Abdiqani Hashi said. Several of them were in critical condition.But the AU mission to Somalia (Amisom) "strongly refuted" allegations that it had killed civilians. Instead, it said its troops encountered a training centre of Islamist group al-Shabaab in the area.Criminal act"In the ensuing gun battle to dislodge the terrorists, as Amisom is mandated to do, five al-Shabaab fighters were killed including one of their notorious leaders, Mohammed Dahir," the mission said in a statement on Tuesday.But a regional official, who did not want to be named, said: "We believe some of those killed were somehow affiliated with al-Shabaab, but innocent civilians were also killed intentionally by the Amisom forces."A legislator representing Merca in the Somali parliament condemned the killings as a "criminal act"."The mandate of AU troops in Somalia is not to target and kill civilians. Instead, they must protect civilians from terrorist attacks," Abdullahi Abshirow said.A police source earlier said the alleged killings could have been an act of revenge after unidentified bombers destroyed an AU water tank the previous evening.The AU has more than 20 000 troops in Somalia to help the government battle al-Shabaab. Somali and AU troops are engaged in an offensive to dislodge al-Shabaab from locations it still holds in the south and centre of the Horn of Africa country.