Some of the web's biggest destinations for watching videos have quietly started using automation to remove extremist content from their sites.

The move is described as a major step forward for internet companies eager to eradicate violent propaganda from their sites.

They are under pressure to do so from governments around the world as attacks by extremists proliferate, from Syria to Belgium and the United States.

YouTube and Facebook are among the sites deploying systems to block or rapidly take down Islamic State videos and other similar material.