Australian Prime Minister Tony Abbott on Tuesday warned of a heightened level of "terrorist chatter" in the aftermath of the fatal Sydney cafe siege and said another attack was considered likely.

Australian Prime Minister Tony Abbott on Tuesday warned of a heightened level of "terrorist chatter" in the aftermath of the fatal Sydney cafe siege and said another attack was considered likely.

"A briefing from the security agencies today indicated that there has been a heightened level of terrorist chatter in the aftermath of the Martin Place siege," he said following a meeting of his National Security Committee.

"We don't know when and how an attack may come, but we do know there are people with the intent and the capability to carry out further attacks."

Gunman Man Haron Monis, who had a history of extremism and violence, took 17 hostages in the city's financial heartland last week, unveiling an extremist flag and demanding to talk to Abbott.

He was killed as armed police stormed the eatery after 16 hours. Two hostages also died -- mother-of-three Katrina Dawson, 38, and 34-year-old Lindt cafe manager Tori Johnson -- while several others were injured.

Abbott did not raise Australia's terror alert level beyond its current high, but cautioned that "at this level, an attack is likely".

"I am alerting people to the fact that the terror threat remains high and as you all understand, at this level, an attack is likely," he said.

"That's why it's important that people remain alert and aware as well as reassured that our police and security agencies are doing everything they humanly can to keep us safe."

Australia raised its threat level in September as it carried out a series of counter-terrorism raids across Sydney and Brisbane following a flow of its nationals to Iraq and Syria to fight with ISIL and other terrorist groups.

Australia has not officially branded the Sydney seige as a terrorist attack.

Abbott said we was "not going to speculate" about any specific future attacks, emphasizing only that "there has been a heightened level of chatter amongst people who we would normally think of as terrorist sympathizers".