* De Mistura hopes Syrian opposition takes stock and unifies
* U.N. envoy sees peace talks in Geneva in October/November
* U.N. convoy reaches Douma with Russian help
* "Worst place" in Syria is Raqqa, 25,000 trapped - Egeland
By Stephanie Nebehay
GENEVA, Aug 17 (Reuters) - The United Nations hopes for a "serious negotiation" between the government and a still-to-be-formed unified Syrian opposition in October or November, U.N. Special Envoy Staffan de Mistura said on Thursday.
After seven previous rounds that have failed to persuade the adversaries to hold face-to-face talks, let alone make progress, the veteran diplomat outlined a new timetable for talks to end the six-year war in which hundreds of thousands of people have been killed.
He said he expected a meeting in October, possibly in Riyadh, among the three opposition delegations "to take stock of the realities on the ground", with a view to consolidation.
Before that, he planned a brief U.N. round of diplomacy with the oppositions and government around mid-September in Geneva.
"We may be eventually focusing basically on the agenda for the real substantive talks that we hope will take place in October," he told reporters.
"Regarding the (Syrian) government, we are counting very much on Russia, on Iran, on anyone who has got major influence, and on the government of Syria to be ready finally to initiate when they are invited to Geneva, a genuine, direct negotiation with whatever (opposition) platform comes out."
The main opposition is the High Negotiations Committee (HNC) but there are two other dissident groupings, the "Moscow" and "Cairo" platforms. The two, much less opposed to President Bashar al-Assad than the HNC, each comprise some activists but do not control territory or have strong links with armed groups.
De Mistura also said that a letter from Russian Defence Minister Sergei Shoigu in early August had paved the way for Russian military police to be staged along the route of a U.N.-Red Cross convoy which reached the besieged rebel-held town of Douma, in eastern Ghouta near Damascus, on Thursday.
The successful convoy, the first since May, consisted of 50 trucks carrying relief items for 35,000 people. "We are seeing just today one of those examples, and I am expecting and hoping that this will become a pattern," De Mistura said.
Jan Egeland, U.N. humanitarian adviser, said that the convoy to Douma, one of 11 besieged areas, was "hugely symbolic".
"It took a lot of effort, a lot of negotiations and a lot of help from Russia and others to make it happen," he said.
Assad is extending his military advantage, helped by the capture of swathes of territory from Islamic State with the help of his Russian and Iranian allies.
IS has also lost ground to the U.S.-backed Syrian Democratic Forces (SDF), dominated by the Kurdish YPG militia. U.S.-backed forces have Islamic State fighters surrounded in central Raqqa, a Syrian Kurdish commander said last week.
"The worst place probably today in Syria today is the part of Raqqa city that is still held by the so-called Islamic State," Egeland said.
"We reckon there to be 20,000 or 25,000 civilians there. They are encircled by the SDF fighters and they are used seemingly as humanitarian shields by the Islamic State."
"We're therefore urging the coalition, the SDF whom we can deal with, to allow, as much as they can, people to escape and come out and to avoid civilian casualties, of which there have been many," Egeland said. (Reporting by Stephanie Nebehay; Editing by Mark Trevelyan)
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