AFP/ Riyadh

Syria's main opposition groups agreed at unprecedented talks Thursday to negotiate with President Bashar al-Assad's regime but insisted he must step down at the start of a political transition.

"The participants are ready to negotiate with representatives of the Syrian regime based on the Geneva 1 communique... within a specific timeframe that would be agreed on with the United Nations," said a statement issued after two days of talks between a range of armed and political opposition groups in the Saudi capital Riyadh.

But the opposition groups insisted that "Bashar al-Assad and his aides quit power with the start of the transition period" set out last month in Vienna by top diplomats from 17 countries.

The Geneva communique is a document agreed at a peace conference in 2012 that drew up baselines for a Syria peace deal including the formation of a transitional governing body with executive powers.

The opposition groups in Riyadh also called on the United Nations and international community "to force the Syrian regime to implement goodwill measures before the negotiation process begins".

The measures include the ending of regime sieges on towns and districts to allow the entry of humanitarian aid, a halt to executions, the release of political detainees and the creation of conditions to allow for the return of refugees.

The regime should also end "the forced displacement" of citizens and the dropping of "barrel bombs on civilian gatherings," it said.

Under the agreement, opposition groups have formed a "supreme committee for negotiations" based in Riyadh which will act as a reference for their negotiating team, whose members the body itself will choose.

The composition of the committee was not immediately clear.

Suhair Atassi, a member of the National Coalition, the main opposition grouping, told AFP that the agreement represented "a unified vision for the settlement process".