Syrian opposition groups say they are ready for UN-sponsored negotiations with representatives from the government, but insist that President Bashar al-Assad has to step down.

"The aim of the political settlement is to create a state based on the principle of citizenship without Bashar al-Assad or figures of his regime having a place in it or any future political arrangements," the group of rebel factions said in a statement issued at the end of two-day talks held in Saudi Arabia's capital city, Riyadh.

"Participants have insisted that Bashar al-Assad and his aides quit power with the start of the transition period," the statement added.

Hosted by the Saudi Arabia, the unprecedented talks between more than 100 representatives from armed and political opposition groups tried to unite the factions fighting against the Syrian government in the bloody conflict in which more than 250,000 people have been killed.

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Monzer Akbik, a member of the National Coalition opposition bloc, said the meeting agreed to set up a 25-strong leadership group, including six coalition members, six from rebel factions, five from the NCB and eight independent figures.

"These are representatives of all the opposition factions, political and military, and they are going to be the decision makers in terms of the political settlement," said Akbik.

He was speaking from the United Arab Emirates after being briefed on Thursday morning's talks. A separate negotiating team of 15 members would also be appointed, he told Reuters.

The groups said they are prepared to meet the government in the first 10 days of January.

'Progress'

US Secretary of State John Kerry, speaking in Paris, said the Riyadh talks had made progress "but we have some tough issues to get over."

A possible December 18 meeting to advance the Syrian peace talks in New York is "not locked in yet", Kerry added.

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.

Saudi Arabia is a main backer of the rebels along with Turkey and Western countries. Iran and Russia support Assad, and say it is up to the Syrian people to decide when the leader should leave.

Moscow 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.

READ MORE: Key Syrian opposition group pulls out of Riyadh talks

The Riyadh meeting was meant to bring as " broad a cross-section of Syrian opposition groups as possible" to the table, according to Adel Al-Jubeir, Saudi Arabia's foreign minister.

Ahrar al-Sham quits... and returns

One key Syrian opposition group, Ahrar al-Sham, quit the meeting on Thursday but is reported to have returned and signed the statement.

The armed rebel group had earlier said it withdrew because the meeting "had given top key roles to the National Coordination Committee and other figures who are considered supporters of the regime".

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Ahrar al-Sham, a faction which Russia does not want to negotiate with, said in a statement that the conference "did not consider some of the key issues they, and other groups, wished to include in the discussions and that [the organisers] refused to emphasise the Muslim identity of our Muslim [Syrian] people".

Ahrar al-Sham, which has a controversial record in terms of alleged human rights abuses and links to al-Qaeda, has been described as a "radical" and "sometimes even a terrorism" group by Russian and Iranian officials.