SYDNEY, Australia — An Islamic State fighter who posted a photo of a beheaded Syrian soldier online has become the first dual national to be stripped of his Australian citizenship under antiterrorism laws, according to local news reports.

A spokeswoman for the Australian Immigration Department said in an interview that someone had been stripped of citizenship under antiterrorism laws but declined to divulge the person’s name. But a leading newspaper, The Australian, identified the person as Khaled Sharrouf, 35, reporting that a secret panel of intelligence officials, police officers and lawyers had revoked his Australian citizenship this year.

“If there was a poster boy for stripping citizenship, it’s Khaled Sharrouf,” said Rodger Shanahan, a research fellow at the Lowy Institute for International Policy in Sydney.

Australia, a longtime ally of the United States in its fight against Islamic militants in Afghanistan, Iraq and Syria, is on alert for potential attacks by Islamic extremists, including fighters returning from conflict zones. About 100 Australians are believed to be fighting or engaged with terrorist groups in Syria and Iraq, according to the Attorney-General’s Department. Mr. Sharrouf is believed to still be in Islamic State territory.