An Australian teenager has been arrested in Lebanon after local authorities suspected him of trying to enter Syria to join Islamic State, officials say.

The 18-year-old from Sydney was caught by the Lebanese Internal Security Forces (ISF) two days before his planned departure to Syria, according to an official statement.

Since his arrest he has been divulging details of Australian ISIS fighters, as well as Australian supporters and financial backers of the terror group.

The Department of Foreign Affairs and Trade has confirmed to Fairfax Media that it is providing consular assistance to an Australian man in Lebanon, but owing to privacy rules "will not provide further comment".