A fighter of holding the Islamic State flag in Iraq.

Church-goers in Sydney's west have been left shaken after a stranger shouted death threats from a car bearing the Islamic State flag.

The car drove past Our Lady of Lebanon Church at Harris Park on Tuesday and witnesses claim it had a flag similar to those brandished by Islamic State jihadists hanging out the window.

A priest from the church told AAP the people in the car threatened to "kill the Christians" and slaughter their children.

"They were strong words and people were scared of what they saw," he said.

Witnesses told police there was a small triangular flag placed out the window with Arabic words similar to "there is only one god and Muhammad is the prophet".

Rosehill police Inspector Brian Jackson confirmed "some threats were made in regard to some people" near the church.

Maronite Catholic parish priest Monsignor Shora Maree contacted police ahead of the church's 7pm mass on Wednesday night.

Officers were sent down to patrol the Harris Park church while hundreds took part in mass inside.

It's understood detectives are looking into who is behind the threats.

Morning news break – September 18

