Syria peace talks due to start this week in Geneva are based on the broad mandate of a U.N. resolution that asks the U.N. mediator to hold talks on a "political transition process", an official involved in the talks said on Tuesday.

Last week the United Nations appeared to back away from using the phrase "political transition", which is understood by the opposition to mean a removal of President Bashar al-Assad or at least an erosion of his powers.

Michael Contet, chief of staff of Special Envoy for Syria Staffan de Mistura, told a regular U.N. briefing that de Mistura was putting the final touches to arrangements for the talks.

"The invitations as well as the substantive agenda for the negotiations are all based on the wide scope of Security Council resolutions, in particular (U.N. Security Council Resolution) 2254, which is our main guidance in this process.

    "The second operative paragraph of 2254 requests the special envoy to convene formal negotiations on the political transition process."

The talks would revolve around three sets of issues, which 2254 mandates de Mistura to mediate on: establishment of a credible, inclusive and non-sectarian governance; a process for drafting a new constitution; and free and fair elections under U.N. supervision.

Contet declined to take questions.