SYDNEY (REUTERS) - A Melbourne teen arrested after the police found explosives at his home pleaded guilty on Monday (Dec 14) to a terrorism-related charge, highlighting concerns about youth radicalisation following last week's arrest of a 15-year-old Sydney boy in police raids.

The 17-year-old was planning an attack using improvised explosive devices, the police said in May when they raided his home in Greenvale, 20km north of Australia's second city, Melbourne.

The boy, who could not be identified because of his age, pleaded guilty to a single charge of "engaging in an act in preparation for, or planning, a terrorist act", the Australian Associated Press reported from the courtroom.

Prosecutors dropped two other charges following the guilty plea, the AAP reported. Victoria state County Court officials could not be reached immediately for comment.

Australia, a staunch ally of the United States and its battle against Islamist militants in Iraq and Syria, has been on heightened alert for attacks by home-grown radicals since last year.

Last Thursday, police said they had charged a 20-year-old man and a 15-year-old boy with conspiracy to attack government buildings after they were arrested during early-morning raids by counter-terrorism police in Sydney.

Three other men, all of them already in custody on terrorism-related offences, were also charged as part of the same operation.

The police have described as "disturbing" the high number of young Australians charged with or involved in violent actions. The matter has raised concerns about how well the government is dealing with at-risk youth in the Muslim population.

Just weeks before the 17-year-old boy was arrested, five other Melbourne teenagers were arrested over the planning of an unrelated Islamic State in Iraq and Syria (ISIS)-inspired terrorist attack against the police on the national Anzac day holiday.

In September 2014, police shot dead a teenager in the southern city of Melbourne after he stabbed two counter-terrorism officers. Three months later, two hostages were killed when the police stormed a central Sydney cafe to end a 17-hour siege by a lone gunman, who was also killed.

A 15-year-old boy shot and killed an accountant at the police headquarters in the Sydney suburb of Parramatta in October and was then killed in a gunfight with the police outside the building.