SYDNEY (Reuters) - Australian police said on Tuesday (June 13) a third man had been charged with supplying a firearm used in a deadly siege in last week, which Prime Minister Malcolm Turnbull has called an "act of terrorism".

Victoria state police said in a statement on their website that the 47-year-old male suspect faced three charges, including possession of a firearm unlawfully, possessing ammunition without a permit and being an unlicensed firearm dealer. He appeared in court earlier in the day.

A 25-year-old man was arrested and charged in relation to the firearm on Monday (June 12) and a 30-year-old man was charged on Friday (June 9) in connection with the same matter, police said.

The deadly siege took place on June 6 in Australia's second-largest city of Melbourne, with police shooting dead gunman Yacqub Khayre after he killed a man in the foyer of an apartment block and held a woman hostage inside.

Turnbull had said police were treating the siege, during which three officers were injured, as an "act of terrorism"after a claim by the Islamic State in Iraq and Syria (ISIS) group that one of its fighters was the gunman responsible.

Australia, a staunch ally of the United States and its action against the ISIS militant group in Syria and Iraq, is on high alert for attacks by sympathisers of the group.

"The national terror threat level remains at 'probable' and we are not immune from the global impact of the conflicts in the Middle East and the instability around the world," Turnbull told Parliament.

Australia has also signalled a drive to reform parole laws as a result of the Melbourne siege, including a ban on parole for violent offenders who have any links to extremism.

Khayre was on parole for a violent break-and-enter offence.