SYDNEY, Australia - A teenager was arrested on Tuesday after Australian police alleged he had been planning to launch a terrorist attack in Sydney.

The 18-year-old was expected to be charged with planning a terrorist attack, the Australian Federal Police said. They said another charge - preparing to enter a foreign country to engage in hostile activities - was expected to be filed in connection with an earlier attempt by the teen to travel to Syria. If convicted, he could face a maximum sentence of life in prison.

"The planning was occurring now and we would say that an attack was probably imminent," New South Wales Police Deputy Commissioner Catherine Burn told reporters.

The teen had been making arrangements to get a firearm, Australian Federal Police Acting Deputy Commissioner Neil Gaughan said. Officials believe he was working alone, and his arrest means there is no longer any threat to public safety linked to the alleged plot, Gaughan said.

Burn and Gaughan declined to detail what the teen's alleged plans involved, including what sites were the proposed targets.

The teen, who had been on authorities' radar since last year, was stopped at Sydney airport in February when he attempted to travel to Syria, Gaughan said. His passport had been cancelled and he was prevented from leaving the country.

Since then, the teen had been under police surveillance, Attorney-General George Brandis told reporters.

Australia's government raised the country's terror threat level in 2014 in response to the domestic threat posed by supporters of the Islamic State group. Police have since conducted dozens of raids they say have been aimed at thwarting multiple terror plots around Australia, including an alleged plan to attack government buildings and a naval base in Sydney.

Many of those arrested have been teenagers, including a 16-year-old who was charged with plotting a terrorist attack on an Australian Veteran's Day ceremony in April.

"We are still seeing people planning and preparing for such attacks and unfortunately, that group of people are getting younger and younger," Burn said.