SYDNEY - Australian police said on Tuesday they were treating a siege in Melbourne as an "act of terrorism" after a claim by Islamic State that one of its fighters was the gunman responsible.Police on Monday killed in a shootout the man they said had held a woman hostage inside an apartment building in Melbourne.Islamic State said via its news agency, Amaq, that the attack was launched because of Australia's membership in a US-led coalition against the militant group."We are treating it as an act of terrorism," Victoria state police Commissioner Graham Ashton told the Seven television network on Tuesday. "We believe this person was there with those sorts of intentions, albeit we don't know whether it was something planned or something spontaneous at this stage."Police were also investigating a telephone call made to the Seven newsroom during the siege, where, according to the network, a male caller said the attack was related to Islamic State.Australia, a staunch ally of the United States and its escalating action against Islamic State in Syria and Iraq, has been on high alert for attacks by home-grown militants returning from fighting in the Middle East or their supporters.Police rescued the woman taken hostage in Melbourne after a standoff lasting longer than an hour in the beachside suburb of Brighton.Three officers were wounded in the shootout which ended the siege, and a second man, in addition to the hostage-taker, died from a gunshot wound, police said.The country has been on heightened alert for attacks by homegrown radicals since 2014. In December of that year a gunman killed two hostages during a 17-hour siege at a popular Sydney cafe.Since September 2014, police have foiled a dozen terrorist plots and made 63 arrests, Australian Prime Minister Malcolm Turnbull said on Monday.