MANILA, Sept 21 (Reuters) - The Philippine military is preparing to try at least 13 soldiers for their alleged role in the murder of left-wing activists, the first cases to be brought before new tribunals set up to hear human rights complaints. The military chief said on Friday the soldiers were undergoing pre-trial investigation for four cases of murder, including that of a female radio talk show host on the southern island of Mindanao, since 2005. "We want to be thorough about it," General Hermogenes Esperon told reporters, adding it would take some time before the cases would go to trial. "While we want to be quick on this, due diligence and the rules of the procedures must be observed. We don't want to pin down anybody based only on suspicions." Human rights groups say more than 800 people have been killed since President Gloria Macapagal Arroyo came to power in 2001 through an army backed-popular uprising. A U.N. special rapporteur on extrajudicial killings in February said the military was responsible for many of the deaths and nearly 200 disappearances of left-wing activists, farmers, unionists and community organisers.