Authorities have warned that suspected terrorists in Australia are becoming younger, with a 12-year-old boy now under investigation by security agencies.

Australian Federal Police Commissioner Andrew Colvin told Australian Broadcasting Corp. television late Wednesday police are "shocked that a 12-year-old is on police radar for these types of matters,"

Colvin was commenting ahead of an urgent national summit of federal and state law enforcement officials convened by Prime Minister Malcom Turnbull in the national capital Canberra on Thursday to counter violent extremism.

It follows the shooting death outside a Sydney police building this month of accountant Curtis Cheng by 15-year-old Farhad Jabar, who was later shot dead by police.

ABC reported that the 12-year-old boy was the youngest of 18 suspected extremists named in a court document in March.