* Opposition to raise demands with U.N. envoy
* Officials say demands in line with U.N. resolution
* Saudi-Iranian tensions add to complications (Adds quotes and details)
By Tom Perry and Suleiman Al-Khalidi
BEIRUT, Jan 4 (Reuters) - Syria's opposition wants to see confidence-building steps from Damascus including a prisoner release before negotiations due this month, officials said on Monday, a demand that could complicate efforts to start the talks.
Opposition leaders including rebels plan to deliver that message to the U.N. envoy for Syria, Staffan de Mistura, when they meet him in Riyadh on Tuesday, three opposition officials familiar with preparations for the meeting told Reuters.
The United Nations said last month it aimed to bring together the warring parties on Jan. 25 in Geneva to begin talks aimed at ending nearly five years of conflict that has killed an estimated 250,000 people.
The effort aims to build on a U.N. Security Council unanimously approved on Dec. 18 endorsing an international road map for a Syria peace process with peace talks due to start in early January.
The opposition body meeting in Riyadh groups political and armed opponents of President Bashar al-Assad. It was formed last month as part of a Saudi-backed effort to get the opposition ready for talks. Riyadh is a major backer of the opposition to Assad, who is in turn an ally of its regional rival Iran.
George Sabra, a member of the political opposition, said the talks must be preceded by "real steps on the ground that express not only good will but also confidence-building measures such as releasing political detainees and stopping the bombardment of towns and cities by heavy artillery and jets".
A second official said the opposition leaders would tell de Mistura "they can't go to negotiations without Assad doing something serious such as a ceasefire or releasing detainees".
The third said: "There will be no negotiations before the issuance of implementation of good-will measures". He said they must include a halt to bombardments and lifting of blockades imposed by the government on rebel-held areas.
The officials said their demands were in line with the Security Council resolution, specifically articles calling for the release of arbitrarily detained people, an immediate halt to attacks on civilians and unhindered humanitarian access.
The U.N. Security Council resolution appeared to mark a rare show of international unity over a crisis that has divided major powers. The United States says Assad must leave power, while Russia has sent its air force to Syria to help shore up his government's control over western Syria.
The outlook for diplomacy on the Syrian conflict has been further complicated by an escalation in tension between Saudi Arabia and Iran following the execution of a Shi'ite cleric in Saudi Arabia.
The European Union's foreign policy chief Federica Mogherini warned Iran's foreign minister on Sunday that renewed tension between Shi'ite Iran and Saudi Arabia's Sunni monarchy could wreck efforts to find a political solution for the crisis in Syria.
Sabra, speaking to Reuters from Riyadh, added that the political process could not move ahead in isolation from the war still raging on the ground.
"We don't want to get into negotiations for the sake of negotiations we want to get into negotiations to reach a real political solution," he said. (Writing by Tom Perry; Editing by Tom Heneghan)