Syrian refugees, pictured here in Lesbos, Greece, have been streaming out of their war-torn homeland for years, often going to Lebanon, Jordan, or to European Union countries. More than 12 million people have fled from Syria since violence erupted there more than four-and-a-half years ago, the United Nations says. Agence France-Presse/Getty Images

The number of people who have fled their homes in war-torn Syria has topped 12 million, the United Nations reported Friday, as armed conflict continues to rage in the nation. This figure has accounted for a 20 percent population decrease since the conflict began almost four-and-a-half years ago.

Syria's civil conflict began in 2011 as pro-democracy protests broke out against President Bashar Assad, the Baath Party leader who was accused of corruption and having tortured dissenters. In the year that followed violent clashes between pro- and anti-government groups escalated.

Beginning in September 2014 and throughout this year, Sunni extremists -- who declared themselves members of the Islamic State group -- capitalized on the chaos to take over large swathes of Syria in an attempt to impose rule by Shariah law, as the nation continued a descent into full-scale war.

"Syrians now have two choices: either to return and die in their country or to emigrate," said Mohammed al-Hariri, a Syrian who has been living in one of Jordan's refugee camps, Agence France-Presse reported Friday.

At least 4 million people have become official refugees, a status that a 1951 United Nations convention defines as a person who is forced to leave his or her country because of systemic violence or persecution. Another 7.6 million have been displaced from their homes but have stayed inside Syria. Turkey and Lebanon have each taken in at least 1 million refugees.

Thousands of Syrian refugees have attempted to flee to Europe, entering through Greece on rubber dinghies from Turkey. Eastern Greek islands such as Lesbos have been some of the most common points of entry for Syrians departing from Turkey, a nation only 6 kilometers from Greece. Crossing the Aegean Sea from Turkey to Greece takes only two hours, but passengers have been charged as much as 2,000 euros ($2,240) to make the dangerous nighttime journey.