UK law enforcement issued an urgent warning on Monday about criminal gangs overseas targeting British teenagers in so-called sextortion scams online.

"Sextortion" is a type of blackmail where criminals threaten to release nude or semi-nude photos of someone -- either real or fake -- unless the victim pays them.

Cases -- particularly against teenage boys -- are soaring worldwide, according to non-governmental organisations and police.

Britain's National Crime Agency (NCA) issued an alert to hundreds of thousands of teachers telling them to be aware of the threat their pupils might face.

It said that gangs based in West Africa and Southeast Asia are targeting young people in the west, particularly Britain, the United States, Australia, New Zealand and Canada.

The scammers often pose as another young person, making contact on social media before moving to encrypted messaging apps and encouraging the victim to share intimate images.

They often make their blackmail demands within an hour of making contact and are motivated by extorting as much money as possible rather than sexual gratification, the NCA said.

"A lot of victims feel responsible but we need them to know this is absolutely not the case. You are not to blame and help and support is available," said James Babbage, NCA's director general for threats.

Global cases more than doubled from 10,700 in 2022 to 26,700 last year, according to the US National Center for Missing and Exploited Children, an NGO.

All age groups and genders are being targeted but a large proportion of victims are boys aged between 14 and 18.

Nine out of 10 of the UK cases dealt with by the Internet Watch Foundation (IWF), a UK watchdog, in 2023 concerned male victims, the NCA said.

"Sextortion has become a major threat online in the last few years," said Susie Hargreaves, chief executive of the IWF.

"This alert to schools is an absolutely crucial intervention in stemming this epidemic which has already ruined so many young lives," she added.

The NCA issued guidance to teachers about spotting abuse signs, supporting young people and encouraging them to seek help.

It also instructed parents and carers on how to talk to children about sextortion and how to support them if they become a victim.

Advice includes not paying, stopping communication and blocking the offender. Report incidents to police and try to avoid deleting anything that could be used as evidence, the NCA added.

Murray Dowey, 16, took his own life at home in Scotland last year after an online blackmailer posing as a young woman threatened to leak intimate details about him to everyone in his contact list.

His mother told ITV News there "has to be consequences" for those who targeted their son, and to deter others from doing the same.

Separately, British police opened an investigation earlier this month following a complaint by an MP who was among a dozen men targeted in a honey-trap sex-messaging scandal. Experts have suggested it could be the work of criminals seeking compromising material.