SINGAPORE - A second propaganda video by terror group Islamic State in Iraq and Syria (ISIS) featuring Singaporean Megat Shahdan Abdul Samad has surfaced.

This time, he goes beyond rallying fighters to join ISIS, which is based largely in the desert along the Iraqi-Syrian border. In the graphic video, he and two other men dressed in military fatigues are shown executing three men by shooting them.

Shahdan, 39, was last known to have appeared in a 3 1/2 minute ISIS clip in September, in which he praised East Asian fighters, called for extremists to join the terror group's efforts in East Asia or the Middle East, and challenged Britain's Prince Harry to a fight.

The newly-surfaced clip, which has been shared on social media and messaging groups, is over eight minutes long. It shows footage of bombings and vehicle attacks, interspersed with scenes at places like stadiums and parties.

Towards the end of the video, Shahdan is featured speaking in English, telling followers to strengthen themselves and "slay the enemies of Allah wherever you can find them", before shooting a kneeling man.

Security experts who spoke to The Straits Times believe the video to be authentic. It is believed to have been first posted on Friday (Dec 29).

When contacted, the Ministry of Home Affairs said it has "no further updates on this video".

It is the first known video of a Singaporean participating in an execution for ISIS, which suggests that Shahdan is in some kind of leadership role, said S. Rajaratnam School of International Studies (RSIS) senior analyst Jasminder Singh.

Dr Kumar Ramakrishna of RSIS said this latest development drives home in a very strong way the dangers of being indoctrinated with extremist ideology.

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"Singaporeans may think we are educated, but even people in our particular day and age, given exposure to extremist ideology, can be influenced to such an extent... where they can so dehumanise other people not seen to be part of their circle that they can just murder them in cold blood," he said.

"It shows why extremist ideology is so dangerous and has to continue to be countered," he added.

Another man whom experts believe to be also Southeast Asian but not from Singapore is also featured in the video. This suggests that the Southeast Asian fighters have most likely regrouped, as Shahdan did not appear with other Southeast Asians in the previous video, said Mr Singh.

Shahdan, who had been a secret society member in Singapore with a string of drug and criminal convictions, went to the Middle East to work in 2014, where he was believed to have been radicalised. He later made his way to Syria to join ISIS.

Home Affairs and Law Minister K. Shanmugam said after the previous video was released that the Internal Security Department had been keeping tabs on Shahdan.

The new video is specifically timed to incite attacks at New Year celebrations, said Professor Rohan Gunaratna, head of the International Centre for Political Violence and Terrorism Research in Singapore.

"At a time when ISIS is shrinking in its battle space in Iraq and Syria, it is sending a message that it is still capable of mounting attacks and terrorising the public," he said. "With its core depleting and global expansion, ISIS will focus on propaganda in the coming months and years."