Transcripts of Besim's alleged exchanges with the boy have emerged in Australian media reports.
According to the reports, the alleged plot was uncovered by secret surveillance of Besim - a friend of another teenager Numan Haider, who was suspected of threatening Australian prime minister Tony Abbott before he was shot dead.
Communications allegedly revealed Besim was planning to target police and intelligence officers with firearms and "a massive machete".
The British boy urged him on and suggested videotaping the attack and sending it to an IS recruiter.
He also instructed Besim to "start dressing like a Kuffar", and asked if he was "willing for a bullet to go through you".
During another conversation days later, the British boy told Besim not to underestimate the "difficulty of beheading a person" and advised him: "U gotta be a lion especially that ur doing it in public."
He is being held in an unidentified youth detention centre in the North West of England.
Sentencing was adjourned to September 3 at Manchester Crown Court.
