Australian police say they believe a 15-year-old who killed a police worker in Sydney had links to terrorism.

The teenage boy was killed by New South Wales police on Friday after he gunned down the worker at close range outside the state police headquarters.

Police say the boy, who was of Iraqi-Kurdish heritage, probably acted alone.

In a separate case in Britain, another 15-year-old boy on Friday was given a life sentence for plotting to behead police officers at a Melbourne parade.

The Blackburn teenager - who sent thousands of online messages to an alleged Australian jihadist - will serve at least five years and will only be released once he is no longer considered dangerous.

'Religious slogans'

The man killed in Friday's incident has been named as Curtis Cheng, a finance worker who had worked for the New South Wales police force for 17 years.

He was shot dead outside his office in Parramatta, a western suburb of Sydney, in what police describe as a targeted attack.

According to some reports, the gunman - whose name has not yet been released - shouted religious slogans before carrying out the attack. He was subsequently killed in an exchange of fire with volunteer police officers.

"We believe his actions were politically motivated and therefore linked to terrorism," New South Wales police commissioner Andrew Scipione told reporters on Saturday.

But he said that the boy's "exact motivation still remains a mystery to us".

The boy, who was born in Iran, was previously unknown to police or intelligence services, he added.

Australia has stepped up counter-terrorism efforts in recent months in the face of what officials say is a growing security threat from radical Islamists.

Recent Australia terror episodes

--December 2014: Iran-born man kills two people after taking customers of a central Sydney cafe hostage for 17 hours

--September 2014: Police shoot dead 18-year-old male who stabbed two counter-terrorism officers in Melbourne

--April 2015: Several teenagers arrested on suspicion of plotting an IS-inspired attack targeting police during Anzac Day

--May 2015: Police arrest a 17-year-old accused of plotting to detonate three homemade bombs

Dozens of Australian nationals are thought to be fighting for Islamic State (IS) militants in Iraq and Syria.

Experts are worried about the effect of returnees - and on those who support them - on security.

Australian Prime Minister Malcolm Turnbull described Friday's incident as "a shocking crime" and "cold-blooded murder".

"It was doubly shocking because it was perpetrated by a 15-year-old boy. And it underlines the importance of families, communities, leaders being very aware of whether young people are becoming radicalised," he told reporters on Saturday.

But he also warned against vilifying the Muslim community for "a very, very small percentage of violent extremist individuals".