GENEVA, Switzerland: The United Nations called on Thursday for a humanitarian pause to allow an estimated 20,000 trapped civilians to escape the Syrian city of Raqqa, and urged the U.S.-led coalition to rein in air strikes that have caused casualties.

"Boats on the Euphrates must not be attacked, people who come out cannot risk air raids when they come out," Jan Egeland, humanitarian adviser on Syria, told reporters in Geneva.

"So now is the time to think of possibilities, pauses or otherwise that might facilitate the escape of civilians, knowing that [Daesh] fighters are doing their absolute best to keep them in place," he 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: "Based on our assessment of what happened in Riyadh we will decide how to move ahead in the future."