* Former ICC chief prosecutor to be named on Friday
* UN rights body extends mandate of war crimes probe
* Inquiry led by Pinheiro has secret list of suspects
GENEVA, Sept 28 (Reuters) - Carla del Ponte, the International Criminal Court's former chief prosecutor, will be named on Friday to join a United Nations investigation into war crimes in Syria, diplomats said.
The inquiry is gathering evidence for possible future trials of figures suspected of committing abuses in the 18-month-old war pitting President Bashar al-Assad's military and police against opposition forces.
Switzerland has proposed that del Ponte, a former Swiss attorney-general, join the panel as a commissioner and an announcement was expected by the end of the day, diplomats said.
The appointment of such an experienced lawyer will be a strong sign of the U.N.'s determination to bring human rights violators to justice.
Her eight years at the Hague-based court were dominated by the pursuit and trial of former Yugoslav President Slobodan Milosevic, who died in 2006 before sentencing.
"She brings strong investigative skills and an ability to better systematise and use the huge amount of information that the commission of inquiry is putting together with a view to prosecution one day," European Union (EU) ambassador Mariangela Zappia told Reuters.
The U.N. Human Rights Council on Friday extended the mandate of the Syria inquiry by another six months and condemned violations by government forces and pro-Assad militias but also by rebel fighters.
The 47-member forum adopted a resolution by a vote of 41 states in favour, with three states - China, Cuba and Russia - against and three abstentions.
Syria's ambassador Faysal Khabbaz Hamoui rejected the text submitted by seven Arab states and backed by Western powers as "highly-politicised and selective".
He accused Islamic "terrorists" of fueling the violence in his country.
The international inquiry led by Brazilian Paulo Pinheiro, set up a year ago, has interviewed more than 1,100 victims, refugees and defectors but has been denied access to Syria.
Earlier this month it expanded its secret list of Syrians suspected of committing war crimes and crimes against humanity, including executions and torture, whom it says should face future prosecution.
Del Ponte met last week in Geneva with Pinheiro to discuss widening the inquiry, diplomats said.
Diplomats said that as well as del Ponte, the council's president, Uruguayan ambassador Laura Dupuy Lasserre, is to name Vitit Muntarbhorn, a Thai expert and veteran U.N. investigator, as fellow commissioners alongside Pinheiro and American Karen Abuzayd.
U.S. envoy Eileen Chamberlain Donahoe told reporters she could not confirm del Ponte's appointment but: "I hope that is what we see and I believe it will be."