Syrian rebel groups have joined forces and agreed to work together in preparation for peace talks with President Bashar al-Assad's government.

More than 100 members of Syria's opposition groups formed a joint team of political and armed rebels to meet the government next month for talks seeking a political solution to nearly five years of conflict, the chairman of a Saudi-hosted opposition conference said on Friday. The statement produced from the meeting in Riaydh stressed that Assad could play no part in any transitional agreement.

This signifies "a big change" on behalf of the opposition, Ibrahim Hamidi, the Syria editor of the Saudi owned al-Hayat newspaper, told the BBC.

However the meeting was overshadowed by protests from the powerful Ahrar al-Sham group. They withdrew from the coalition objecting to what it said was a prominent role given to the mainly Damascus-based political opposition group, the National Coordination Body for Democratic Change, which it said was closer to Assad than to the opposition.

It also said rebel fighters had been under-represented at the talks and their voices largely ignored.

However there are conflicting reports as to the nature of this withdrawal as a copy of the final statement, seen by Reuters, had been signed by the Ahrar al-Sham delegate.

Nevertheless the disagreement highlighted enduring rifts among opposition groups which have bedevilled Western and Gulf Arab efforts to rally enough political and military pressure on the president to force him to step down.

Abdulaziz al-Sager, a Saudi academic who chaired the Riyadh talks, said the opposition would meet government officials in the first 10 days of January, the first such talks in two years aimed at ending a bloody civil war which has drawn in forces from the United States, Russia, Europe and the Arab world.

"There will be a meeting decided by [United Nations envoy Staffan] de Mistura in January. A meeting between the opposition and the Syrian regime to go to a transitional period," he said, in comments translated at a news conference.

"This will take place in the first 10 days of January."

The meeting came amid escalating conflict in Syria, pitting the army and allied militias including Lebanese Hezbollah fighters backed by Iran and Russia against competing rebel and jihadi fighters, who include Arabs and Kurds.

Participants of the conference committed to a political system which "represents all sectors of the Syrian people", and would not discriminate on religious or sectarian grounds - in a gesture towards minority Alawite, Christian and Kurdish populations.

US Secretary of State John Kerry welcomed the declaration. "While this important step forward brings us closer to starting negotiations between the Syrian parties, we recognise the difficult work ahead," he said.

International efforts to resolve the conflict have been lent added urgency by a wave of deadly attacks across the world claimed by the Iraq- and Syria-based Islamic State and by a massive flow of refugees into Europe.

Major powers agreed in Vienna last month to revive diplomatic efforts to end the war, calling for peace talks to start by January and elections within two years.

NO PART FOR ASSAD

The demands that Assad and his lieutenants should play no part in a political transition marked a tougher stance than that of several Western countries which back Assad's opponents. The United States, France and Britain all called for Assad to step down after protests broke out against his rule in March 2011.

Although they all say Assad ultimately must go, they have been less specific about the timing of any departures, indicating that they could accept his staying on in an interim period.

Assad's fate was one of several questions left unresolved at the Vienna meeting last month which was attended by Russia, the United States, European and Middle Eastern countries including Saudi Arabia and Iran, which back opposing sides in Syria.

Saudi Arabia is a main backer of the rebels along with Turkey and Western countries. Iran and Russia support Assad.

Iran has openly criticised the decision by Saudi Arabia to hold the talks, saying they were designed to harm the Vienna process. On Thursday, Deputy Foreign Minister Hossein Amir-Abdollahian said some groups linked to the Islamic State militant group were involved in the Riyadh meeting.

Russia launched air strikes in Syria 10 weeks ago, helping the Syrian army - backed by Iranian troops, Hezbollah fighters and allied militia - to contain rebel advances.

Russia says it is bombing Islamic State militants, who control large areas of eastern Syria and western Iraq, but Western and Arab states which have been carrying out air strikes against Islamic State for more than a year say the Russian jets have mainly hit other rebel forces in the west of Syria.

Moscow's intervention has not swung the war decisively Assad's way and several Western-backed rebel groups, some of whom were represented in Riyadh, have been emboldened by the increased flow of foreign-supplied anti-tank missiles which have helped stem parts of the army's counter-offensive.

Additional reporting by Reuters.