Four teenagers have been arrested after suspicious chemicals and weapons were found on a hillside near a vocational school.

Officers on Wednesday said the four secondary school pupils, aged between 13 and 15, knew each other, with one of them being a suspected triad member. They were detained for allegedly conspiring to make explosives and possessing them, among other offences.

The force said they acted upon receiving a report from the Kwun Tong branch of the Institute of Vocational Education, adding that officers found 90 containers with suspected chemicals at the scene. Police also seized weapons and a number of laboratory equipment like beakers and alcohol burners.

"As for the motives of those arrested, whether it involved anything political or if they were manipulated by others, and whether there were any other accomplices besides the four, these are what the police are extremely concerned about," said senior superintendent Eileen Chung of the Kowloon East regional crime unit.

The force added that suspected explosives were found in the residence of one of the suspects.