BEIRUT — An explosion at a gas station outside Damascus on Wednesday turned a long line of cars waiting for rarely available fuel into a deadly inferno that killed at least 30 people, according to reports from witnesses who blamed the blast on a government airstrike.

The violence came as the United Nations released a study showing that nearly 60,000 people had been killed in Syria’s 22-month-old conflict, a number that is a third higher than estimates by antigovernment activist groups.