GENEVA (Reuters) - The United Nations called on Thursday for a humanitarian pause to allow an estimated 20,000 trapped civilians to escape from the Syrian city of Raqqa, and urged the U.S.-led coalition to rein in air strikes that have caused casualties. Amnesty International said on Thursday that a U.S.-led coalition campaign to oust Islamic State from Raqqa had killed hundreds of civilians, and those remaining face greater risk as the fight intensifies in its final stages.   On Raqqa, our urging today from the UN side to the members of the humanitarian task force ... is that they need to do whatever is possible to make it possible for people to escape Raqqa,  Jan Egeland, U.N. humanitarian adviser on Syria, told reporters in Geneva.  Boats on the Euphrates must not be attacked, people who come out cannot risk air raids when and where they come out,  he said.  Now is the time to think of possibilities, pauses or otherwise that might facilitate the escape of civilians, knowing that Islamic State fighters are doing their absolute best to use them as human shields,  he said. Humanitarian pauses were agreed between the warring sides last December to allow the evacuation of civilians from then rebel-held eastern Aleppo, Egeland said. But he added that the United Nations has no contact with Islamic State fighters who have controlled Raqqa since 2014. Egeland, referring to U.S-backed Syrian Democratic Forces (SDF), said:  There is heavy shelling from the surrounding and encircling SDF forces and there are constant air raids from the coalition. So the civilian casualties are large and there seems to be no real escape for these civilians.  Syrian government forces, backed by the Russian air force and Iran-backed militias, have also been advancing against IS south of the River Euphrates that forms Raqqa city s southern edge.  Inside Raqqa city, on both sides, conditions are very bleak and it is very hard to assist in all areas,  Egeland said. The United Nations is still assessing the outcome of talks held this week in Riyadh between the three Syrian opposition groups - who failed to unite - Ramzy Ezzeldin Ramzy, the U.N. deputy special envoy for Syria, said.  Asked whether Syria peace talks would be held in Geneva in September, he said:  We are waiting to get a full picture as to what happened in Riyadh, and we will have further consultations with the interested parties.  And on that basis a decision will be taken as to when these talks will take place.   