Ishmael Duncan 24, used Snapchat to coerce and threaten his victims, the National Crime Agency (NCA) said.

He was arrested in July 2021 in Lambeth, south London, with incriminating images and chat logs later downloaded from his devices and cloud storage.

Some of his 28 victims were as young as nine and from countries including the UK, Canada, Australia and the US.

But investigators think he contacted nearly 10,000 children - and more than 19,000 indecent images were recovered in total.

The NCA said Duncan would begin by asking if the girls wanted to model for well-known brands, before asking for age and personal details and requesting clothed images.

An interview process would follow, along with legitimate-looking contracts.

He would then ask for topless pictures with the ruse of needing them to assess body shape and overlay potential clothes in editing.

Girls who questioned it were told the photos would be deleted, said the NCA, while others were threatened with blacklisting from modelling work if they refused.

He also used another account to contact some victims, often months later, and threatened to expose the photos unless they sent more.

As well as his modelling scam, Duncan targeted children by pretending to be someone of a similar age.

He blackmailed one 14-year-old girl with learning difficulties who sent him images and offered her $1,000 (£800) to perform a sexual act with her brother.

Duncan was charged with 53 offences, including causing or inciting a child under 13 to engage in sexual activity, blackmail, sexual communication with a child, and possession of extreme pornographic images.

He admitted 42 charges last month and on Thursday pleaded guilty to eight more. The remaining three will lie on file.

Duncan will be sentenced at Inner London Crown Court on 1 December.

The NCA's Martin Ludlow said he had "cruelly preyed on young girls who had dreams of becoming models".

"The threatening messages he sent were chilling and showed his callous disregard for the victims he exploited for his own sexual gratification," he added.