The UN's mediator for the crisis in Syria, Staffan de Mistura, said in a statement on Saturday that "the broadest possible spectrum of the Syrian opposition and others" should attend talks with the government of President Bashar al-Assad in Geneva to begin an internationally-backed process to end the grinding five year war.

In a statement issued by his spokesman, de Mistura's office said he "counts on full cooperation of all the relevant Syrian parties in this process."

"Continuing developments on the ground should not be allowed to derail it," said the statement, which comes a day after the death in an airstrike of Zahran Alloush, the rebel leader of a major Islamic faction that was to participate in the peace talks.

The UN Security Council on December 18 approved a resolution backing a road map drawn up by world powers in Vienna that envisions a ceasefire, negotiations between the Syrian regime and the opposition, followed by an 18-month period to create a unity government and hold elections.

The road map leaves the future of Assad unresolved, with Russia and Iran saying the Syrian people should decide his role, while the West and rebel-backers Saudi Arabia, Turkey and Qatar have maintained that he must go.

The Syrian regime agreed to participate in the talks, but has voiced opposition to sitting down at the table with foreign-backed "terrorist" groups.

Syria and Russia insist that world powers draw up a list of undesirable rebel groups that should be excluded from talks.

However, there has been little consensus thus far on the list to be drawn up by Jordan, except agreement that the al-Qaeda affiliate al-Nusra and the "Islamic State" should be excluded from the talks.

Earlier this month Saudi Arabia convened in Riyadh divided rebel and opposition groups, who agreed to form a 34-member committee to oversee talks and a negotiating team met with regime representatives.

The Riyadh group includes Islamist rebel factions that the Assad regime and its backer Russia would like excluded from the talks.

More than 250,000 people have been killed and nearly 12 million people displaced in the war, which has drawn in dozens of countries in an increasingly complex battlefield.

cw/se (AFP, Reuters)