An Australian teenager who ran away from his country to join ISIL militants Syria has reappeared months later in a video of the Takfiri group vowing to "not stop fighting".

An Australian teenager who ran away from his country to join ISIL militants Syria has reappeared months later in a video of the Takfiri group vowing to "not stop fighting".

The 17-year-old, named in local media as Abdullah Elmir but who calls himself "Abu Khaled", carried a rifle and directly addressed Australian Prime Minister Tony Abbott in the video reportedly posted online, the Sydney Morning Herald said.

"To Tony Abbott, I say this. These weapons that we have, these soldiers, we will not stop fighting," said Elmir, whose family is from the southwestern Sydney suburb of Bankstown.

"We will not put down our weapons until we reach your lands and until we take the head of every tyrant and until the black flag (of ISIL) is flying high in every single land."

A spokesman for the prime minister said in a statement the video showed the threat posed by the ISIL group.

"As the Prime Minister has said on many occasions, ISIL is a threat that reaches out to Australia and our allies and partners," the spokesman said.

"That is why Australia has joined the coalition to disrupt and degrade ISIL in Iraq and is giving our law enforcement and security agencies the powers and resources they need to keep Australia and Australians as safe as possible."

Australia raised its terror threat level in September to "high" after years on "medium" on growing concern about returning militants, while Abbott has warned that those fighting with extremists could face lengthy jail terms if they come home.

Several men were arrested in counter-terrorism raids in September and charged with recruiting, funding and sending fighters to Syria. One of them was last week facing fresh charges of preparing a terrorist attack on home soil.