By Stephanie Nebehay

GENEVA (Reuters) - Arab countries proposed on Friday extending the mandate of U.N. investigators documenting war crimes in Syria and said that more experts were needed for the growing task.

A draft resolution submitted to the United Nations Human Rights Council by a group of Arab states, backed by Western powers, calls for the investigations to carry on for the next six months.

The U.N. body launched the commission in August 2011 after a majority of member states out-voted four countries that opposed it, including Russia and China.

Its current mandate expires at the end of the Council's three-week session next Friday, on September 28 by which time the vote on extending the mandate should have take place.

The investigators, led by Brazilian Paulo Pinheiro, said on Monday they had added more names to a secret list of Syrians suspected of committing war crimes during the 18-month-old uprising against President Bashar al-Assad.

Adding weight to the notion that the list could eventually be used to launch prosecutions, Switzerland has proposed that Carla del Ponte, a former ICC prosecutor, join the inquiry as a third commissioner.

"She has the ideal profile and necessary experience to reinforce the commission and carry out its investigative work," a Swiss diplomat told Reuters this week.

Pinheiro and del Ponte, a former Swiss attorney-general whose eight years at the war crimes tribunal were dominated by the pursuit and trial of former Yugoslav President Slobodan Milosevic, met in Geneva this week to discuss the Syria inquiry, diplomatic sources said.

If the Council extends the inquiry's mandate, the choice of a third commissioner would be left to the Council's president, Uruguay's Ambassador Laura Dupuy Lasserre, who could make the appointment as early as next Friday.

WEST-RUSSIA CONFRONTATION

The resolution was submitted by Morocco on behalf of Kuwait, Qatar, Jordan, Libya, Saudi Arabia and Tunisia after being discussed in informal negotiations this week by all members.

The sponsors hope to convince Russia and China to at least abstain rather than voting against this time, to send a strong signal from the 47-member forum that abuses must stop.

"The whole objective is to get as many people as possible on board, to take the Syria issue out of a West-Russia confrontation," an Arab diplomat told Reuters.

The text asks the United Nations to provide additional resources, including staffing, to allow the inquiry to work "in light of the increasingly deteriorating human rights situation" in Syria.

In the past year, some 20 investigators have helped Pinheiro and co-commissioner Karen AbuZayd interview 1,100 victims, witnesses and defectors despite their lack of access to Syria.

"Karen and I have expressed the need to double the number of investigators," Pinheiro told Reuters earlier this week.

The Arab states are also seeking an eighth condemnation of Syrian government forces and allied militia that investigators say have committed atrocities including a massacre at Houla in May which Damascus blamed on "Islamist terrorists".

The draft resolution strongly condemns crimes by Syrian government forces including "the use of heavy weapons and force against civilians", massacres, executions, torture and rapes.

It also condemns "any human rights abuses by armed opposition groups" fighting to topple Assad.

The conflict has uprooted 1.2 million people within Syria and driven nearly 280,000 Syrian refugees abroad to four neighboring countries which need financial support, it says.

(Reporting by Stephanie Nebehay; Editing by Robin Pomeroy)