Dhaka – Bangladesh has launched a clampdown on social media sites spreading jihadist propaganda after an attack on a Dhaka café in which 20 hostages were murdered, saying the country's young were being radicalised online.Authorities said the deadly siege at an upmarket café popular with foreigners had been an "eye-opener", exposing the role of social media in recruiting young men to jihadist groups."Social media has become a fertile ground for recruiting militants," the head of the telecoms regulator Shahjahan Mahmood told AFP."The attack was an eye-opener for us. They [jihadist groups] attract the young men through social media."The Islamic State group, which has claimed Friday night's attack, has long used social media to recruit fighters and incite individuals around the world to commit terrorist attacks.Mahmood said the Bangladesh Telecommunications Regulatory Commission (BTRC) had ordered YouTube to remove videos of "radical preachings", including those of the firebrand cleric Jashim Uddin Rahmani.He was sentenced to five years in jail last December after his speeches were found to have incited Islamist militants to kill the atheist blogger Ahmed Rajib Haider in early 2013.Radicalised onlineShortly after the café siege, it emerged that several of the Bangladesh attackers were young, tech-savvy men from wealthy families and had easy access to social media.The father of 22-year-old Rohan Imtiaz, one of the suspected attackers killed when commandoes stormed the café, has said he believes his son may have been radicalised online.Imtiaz reportedly posted an appeal on Facebook last year urging all Muslims to become terrorists and quoting a controversial Indian preacher who has been banned in Britain, Canada and Malaysia."He was a practising Muslim. So many people are. Maybe he was radicalised through the Internet," his father Imtiaz Khan Babul told AFP."But I never checked what he was browsing... Someone may have brainwashed him."Bangladesh police issued a stern warning on Wednesday that anyone caught sharing jihadist propaganda online would be punished in the wake of the unprecedented attack in Dhaka."Uploading, sharing, commenting or liking any video, images or speech in the social media such as Facebook, Twitter and YouTube in support of the Islamic State or militancy is a punishable offence," Deputy Inspector General of police A.K.M. Shahidur Rahman said."If anyone is found to have engaged in such activities, tough legal action will be taken against that person."Photos of carnageSurvivors of the café siege have told how the young attackers seized mobile phones from hostages and forced them to provide their passwords so they could send out photographs of the carnage that were quickly disseminated by ISIS.Some reports said they had brought laptops, and ordered café workers to switch on the Wi-Fi.Facebook, Twitter and YouTube are already working around the clock to take down jihadist accounts, but they often pop up again under different names.Experts say ISIS supporters are also turning to smaller social media platforms like Telegram, a messenger app, to disseminate their propaganda.Facebook and Twitter have removed some posts and Mahmood, of the telecoms regulator, said the government would bring in tougher laws to tackle radicalism on social media.Bangladeshi police have set up a special email address and urged people to report any suspicious online activities following last week's attack.Meanwhile, the head of the country's elite security force appealed to Bangladeshis to be vigilant over the use of social media sites as a recruiting tool, and to report missing relatives."If there are any missing family members, please tell us, don't be afraid that law-enforcing agencies will take your sons away," said Benazir Ahmed. "Their lives and other lives can be saved if they are found."