MANILA • The Philippines and Indonesia yesterday started their annual joint patrol of the Celebes Sea, in a bid to stop extremists from Malaysia and Indonesia from going to the Philippines' southern island of Mindanao, where rebels have seized a city.

Major Ezra Balagtey, a spokesman for the Eastern Mindanao Command, said the joint patrol aims to strengthen border security around the Celebes and Sulu seas. "The coordinated patrol... is intended to strengthen the security of the Davao Gulf and the common boundary of the two countries in the southern archipelago, particularly along the Celebes Sea," he said in a statement.

Governments in the region worry that extremists may head to Marawi to reinforce militants linked to the Islamic State in Iraq and Syria (ISIS), who are still holding on, seven weeks after they first seized large parts of the city.

The militants' leaders are also said to be plotting to flee to safe havens in Malaysia and Indonesia, as Philippine security forces close in on their remaining strongholds in Marawi.

The latest joint patrol is the third in the region in a month, following one conducted last week by the Philippines and the United States in southern Philippine waters, where the small but brutal Abu Sayyaf terror group has been abducting tourists, fishermen, sailors and traders.

Joint maritime patrols were also held two weeks ago by Indonesia, Malaysia and the Philippines.

Intelligence reports suggest that as many as 200,000 boat trips a year between the Philippines and Indonesia are unchecked.

The latest annual joint patrol of the Celebes Sea is part of the coordinated patrol held by the Philippines and Indonesia each year.

Personnel from the Philippine and Indonesian navies are now on a medical mission in Sarangani town, in Davao Occidental province. They will set sail tomorrow on two warships, and are expected to be in Manado, Indonesia, by next Wednesday.

Raul Dancel