A loyalist of the Islamic State of Iraq and Syria waves an ISIS flag in Raqqa, June 29, 2014. Reuters/Stringer

An ultimatum reportedly issued by rebels aligned with the Islamic State of Iraq and Syria, or ISIS, has sparked a mass exodus of Iraqi Christians from the rebel-controlled city of Mosul, where they were advised to convert to Islam, pay a religious levy or prepare for death, BBC News said.

An al Qaeda offshoot, ISIS captured large pieces of northern Iraq in June, declared the area to be an Islamic Caliphate and renamed itself the Islamic State.

The Sunni extremist group made the threat Thursday night over mosque loudspeakers and from cars driving through the streets of Mosul, the capital of the Nineveh province, about 250 miles northwest of Baghdad, according to the Wall Street Journal. The ultimatum came just about six weeks after the group seized control of the city.

“We offer them [Christians] three choices. Islam, the dhimma contract -- involving payment of jizya -- if they refuse this, they will have nothing but the sword," the announcement reportedly said, with a deadline set for Saturday afternoon.

The concept of dhimma, established in the 7th century, deals with governing non-Muslims living under an Islamic rule and requires them to pay a jizya, or protection money. This practice was abolished during Ottoman Empire reforms of the 19th century.

Patriarch Louis Sako, a senior cleric in Iraq, told Agence France-Presse that Christian families were fleeing to Dohuk and Arbil in the neighboring autonomous region of Kurdistan. “For the first time in the history of Iraq, Mosul is now empty of Christians,” he said.

Iraqi Christians constitute about 5 percent of the country’s population. And Mosul is the birthplace of Assyrian Christianity, one of the most ancient Christian communities. However, their population in the city has been steadily declining since the U.S.-led invasion of 2003.

“Prior to 2003, the number of Christians in the city had been as high as 60,000, but that dropped to about 35,000 in June this year,” Sako said.

Most of the remaining Christians in Mosul fled the city after it fell to the Sunni rebels.

This latest threat is similar to one ISIS made in the Syrian city of Raqqa in February, when it reportedly demanded Christians make payments in pure gold in exchange for their safety.