Malaysia launched trilateral air patrol operations with Indonesia and the Philippines yesterday to combat militancy in the Sulu Sea.

The operations, part of a tri-nation effort to curb movements of militants linked to the Islamic State in Iraq and Syria (ISIS) in the southern Philippines, mark the first joint air patrol over the dangerous waters.

"Affiliated terrorist organisations and sympathisers have exploited the porous borders and linked up with local terrorist groups, evident following the Marawi City siege," said Malaysian Defence Minister Hishammuddin Hussein.

The joint air patrols over the Sulu Sea follow the announcement in June of trilateral joint maritime patrols in the waters. The three countries said both the joint air and maritime patrols will operate concurrently and together to combat and deter extremists across borders.

"The maritime patrol is working very well. In fact, we've not had any incidents of piracy, kidnappings in the maritime areas of concern between the three countries," said Philippine Defence Secretary Delfin Lorenzana.

Mr Lorenzana added that ISIS fighters in Marawi have dwindled to a small group.

Said Datuk Seri Hishammuddin: "Initiatives such as this must be institutionalised so they (ISIS) don't move from Marawi to other places." He also told reporters at Subang air base that the air patrols would "go on as long as it takes".

A meeting of Asean defence ministers later this month would include talks on whether other Asean nations would participate in the joint air patrols, he added.

Indonesian Defence Minister Ryamizard Ryacudu said the country has received requests from the United States, China and Australia to join the patrols but that it would discuss this with Asean members first. The trilateral air patrols will be conducted on a monthly rotation, with Malaysia taking the first month, followed by the Philippines and then Indonesia.

All three countries are opening the air space within and around the Sulu Sea to facilitate the patrols. "We are opening our air space in the adjoining areas of concern over the Sulu Sea. Not just air but also to ships," said Mr Lorenzana.

Mr Ryamizard said Indonesia also plans to propose an intelligence-sharing initiative among Asean members to deal with the threat of radical militants.

"We would also like to launch a new initiative on intelligence cooperation among Asean countries, called 'Our Eyes'," he said at the news conference.

Also present at the launch was Singapore's Senior Minister of State for Defence, Dr Maliki Osman.