Two Royal Australian Air Force (RAAF) C-130J Hercules aircraft fly above the Sydney Opera House and Sydney Harbour Bridge during a display, in this handout picture released by the Australian Defence Force September 10, 2014. [Photo/Agencies]

SYDNEY -? Australia raised its terror threat level to 'high' from 'medium' on Friday, saying there was an increased likelihood of a terrorist attack at home but stressed it had no knowledge of a specific attack plan.

"Last night the director general of security raised the terror threat to high, consequently today the government is raising the public awareness level to high," Prime Minister Tony Abbott said in a news conference in Melbourne.

"I want to stress that this does not mean that a terror attack is imminent. We have no specific intelligence of particular plots. What we do have is intelligence that there are people with the intent and capability to mount attacks here in Australia."

David Irvine, the outgoing head of the Australian Security Intelligence Organisation (ASIO), has said that the number of Australians returning from fighting with Islamic State and other radical groups pose a growing risk.

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