GENEVA — As the United States and its allies struggled for a course of action to punish the use of chemical weapons in Syria, the United Nations said the number of civilians who had fled to neighboring countries had surpassed two million — a new milestone in what it called “the great tragedy of this century, a disgraceful humanitarian calamity.”

Fear of Western airstrikes in the past week was a factor in an exodus that continued to gather momentum, inflicting acute social strain and political tension on receiving countries, António Guterres, the United Nations high commissioner for refugees, said in an interview in Geneva on Monday.

It took two years of conflict in Syria for the refugee figure to reach one million, but only six more months to reach two million, Mr. Guterres noted. In addition, at least 4.5 million people have been driven from their homes inside Syria by the destruction and violence, meaning that close to one-third of the country’s population has been displaced by the civil war, and about half the population has needed humanitarian aid, Mr. Guterres said, putting Syria’s crisis at a level unseen in recent decades.

About 40,000 Syrians fled to Iraq in the last two weeks of August, and 13,000 arrived in Lebanon in the past week. Over all, close to 5,000 Syrians are leaving every day.