Islamic extremists have reportedly threatened to kill Christians in Jerusalem unless they leave the city by the end of Ramadan.

Flyers emblazoned with the notorious black flag of Islamic State have been distributed in Jerusalem by a group calling themselves "The Islamic State – Emirate of Bayt al-Maqdis [the Holy City, ie, Jerusalem]," the Jerusalem Post reports.

The leaflets threaten all those who "collaborate with the Zionists" and urge Muslims to help identify them.

"Those who work with the Zionists also encourage Muslims to leave their religion and become more secular and open, and they spread evil...They take these Muslims away from us...We know where they are, but we need help to find them all".

The flyers promise that soldiers loyal to Islamic State "will work to kill these people so this country is clean of them and... will clean this country and the Muslim Quarter from these Christians during this holy Ramadan."

"So we tell our Christians and the nonbelievers: Go away now or you will be killed when the Id [al Fitr festival ending Ramadan] is near," the group adds.

"And you will be slaughtered like the sheep. One month is enough for them to go away."

A reporter for Israel's Channel 10 television said the incident is "an absolute reminder that the Islamic State isn't only on the Iraqi border, it's also getting closer to the territories and to Gaza...and also to the areas of Arab Israelis."

However, the authenticity of the leaflets has not been verified. A local told Israel Today: "it may simply be an attempt to intimidate local Christians. I do not fear. We are here in Israel, I do not think that Israel will allow that."