Mar Behnam
Christian monks were forced to flee Mar Behnam, a 4th century monastery in Iraq James Gordon

Islamic militants have stormed and seized a Christian monastery in northern Iraq and expelled its resident monks, according to a cleric and residents.

Islamic State of Iraq and the Levant (Isis) militants stormed Mar Behnam, a 4th century monastery, previously controlled by the Syriac Catholic church and a place of pilgrimage near the majority Christian town of Qaraqosh.

"You have no place here anymore, you have to leave immediately," the militants were quoted as telling the monastery's residents by a member of the Syriac clergy.

The unnamed clergy member said that monks had begged with the fighters to keep some of the building's relics but their demands were refused and they were ordered to leave the monastery on foot.

The Mar Benham monastery, built by Assyrian King Sennacherib II, is a major Christian landmark for pilgrims to pray for healing and fertility.

The incident is the latest crackdown on religious freedom since Isis announced the formation of an Islamic caliphate, straddling the Iraqi-Syrian border.

Isis fighters issued an ultimatum to Christians in the northern Iraqi city of Mosul to convert to their radical form of Islam or be forced to either pay a tax, leave the city or be harmed for refusal to convert.

The city is now reportedly empty of Christians as hundreds of families fled following the ultimatum of death or a historic contract ‒ known as "dhimma"‒ where non-Muslims can receive protection if they pay a fee known as a "jizya".

"We offer them three choices: Islam; the dhimma contract ‒ involving payment of jizya; if they refuse this they will have nothing but the sword," the Isis' statement read.

In Syria, Isis militants continue to battle forces loyal to President Bashar al-Assad and, last week, 700 were left dead in just 48 hours in what activists have said is the worst bloodshed seen since the conflict began.