JAKARTA (Reuters) - Islamic State in Iraq and Syria (ISIS) supporters from Indonesia are travelling to join forces with their counterparts in the Philippines, raising concerns about cross-border violence, Indonesian anti-terrorism officials said on Tuesday (Oct 25).

The authorities in South-east Asia have been on heightened alert since a gun-and-bomb attack rocked the Indonesian capital Jakarta in January and stamped ISIS' presence in the region for the first time. "Some (Indonesian ISIS supporters) are training in the Philippines," said Mr A. Syamsu of the Indonesian counter-terrorism agency. "There's no exact number yet but it could be dozens."

The authorities across the region have in the last year cracked down on ISIS sympathisers attempting to travel to Syria.

That has forced many Indonesian radicals to use sea routes to travel to the Philippines instead making it harder to track their movements, national police chief Tito Karnavian said in an interview last week.

Jakarta-based terrorism expert Sidney Jones, in a report that detailed links between Indonesian, Malaysian and Philippine radical networks, said they are now increasing cooperation, making cross-border violence likely.

"As getting to Syria becomes increasingly difficult for South-east Asian fighters, Mindanao (in the southern Philippines) may be the next best option," Mr Jones wrote in the report.

In June, militants who claimed to be fighting for Islamic State said in a video they had chosen the Philippines' most wanted man, Isnilon Hapilon, to lead their South-east Asian faction.

The video, posted on social media, marked ISIS' acceptance of allegiance from South-east Asian supporters and called for them to launch attacks in the region.

Hapilon is known to be a member of the Abu Sayyaf group based in the Mindanao region and known for kidnappings and extortion. He has a US$5 million bounty on his head from the US State Department for the kidnapping of Americans in 2001.

The authorities in the Philippines say there are a handful of Indonesian and Malaysian militants in Mindanao.

Malaysian police have arrested more than 100 suspected ISIS sympathisers this year and stepped up security in case fighters try to return from the Middle East in the wake of an ongoing offensive in the ISIS stronghold of Mosul, Iraq.

"In Malaysia, there have been a few attempted (attacks) so it wouldn't be surprising if there are more around the region, especially if they were desperate," said counter-terrorism chief Ayob Khan Mydin Pitchay, adding it was possible Malaysians were travelling to the Philippines.