London - Britain's interior minister Amber Rudd will meet US tech leaders in Silicon Valley on Tuesday to try and push back against "ruthless" extremists inciting terror online.Social media sites and web giants including YouTube are among the companies attending a San Francisco forum she is scheduled to take part in.Ahead of her arrival, Rudd said jihadist groups such as the Islamic State were incredibly quick at spreading propaganda online."They are ruthless. They prey on the vulnerable and disenfranchised. They use the very best of innovation for the most evil of ends," she wrote in The Daily Telegraph newspaper.After deadly terror attacks in London and Manchester, British Prime Minister Theresa May announced plans to clamp down on extremist content.Four attacks in Britain have killed 36 people this year. Senior ministers have repeatedly demanded that the world's biggest internet companies do more to suppress extremist content and allow access to encrypted communications.Rudd said the government's non access to end-to-end encryption used by messaging services such as WhatsApp is "severely limiting our agencies' ability to stop terrorist attacks and bring criminals to justice".The interior minister earlier this year made similar pleas to get hold of such data, but writing in the Telegraph she said the government had no intention to ban end-to-end encryption.Prime Minister May also wants internet companies to develop tools to automatically identify and remove harmful material, based on what it contains and who posted it.She would also like to see companies block users who post extremist content, and alert authorities when they identify material that could be harmful.