A man has been arrested by homicide detectives over the sexual assault and murder of Sydney teenager Mahmoud Hrouk.

The 22-year-old from Fairfield East was arrested at Fairfield police station at 3pm on Thursday and was expected to be charged with murder and aggravated sexual assault.

It is understood that the man was an associate of Mahmoud and was arrested while reporting to the police station as part of his bail conditions for an unrelated offence.

The body of 16-year-old Mahmoud was unrecognisable when discovered in a derelict house at Fairfield East on May 17.