geneva civilians catch battle syrian city raqqa pay unacceptable price attack force may contravene international law intense air strike top unite nations human right official say thursday u.s.-led coalition seek oust islamic state raqqa syrian government force back russian air force iran-backed militias also advance city 20,000 civilians trap raqqa jihadist fighters hold human shield world body say u.n. high commissioner human right zeid ra ad al-hussein say office document 151 civilian deaths six incidents alone august due air strike ground-based attack give extremely high number report civilian casualties month intensity air strike raqqa couple isil use civilians human shield deeply concern civilians protect time pay unacceptable price force involve battle isil lose sight ultimate goal battle zeid say statement ... attack force may fail abide international humanitarian law principles precautions distinction proportionality say u.s.-led coalition say conduct nearly 1,100 air strike near raqqa month 645 july u.n. statement say russia air force report carry 2,518 air strike across syria first three weeks august add meanwhile isil fighters continue prevent civilians flee area although manage leave pay large amount money smugglers zeid say report smugglers also publicly execute isil u.s.-led warplanes wednesday block convoy islamic state fighters families reach territory group hold eastern syria strike comrades travel meet coalition spokesman say heavy civilian casualties raqqa air strike u.n