MANILA (Reuters) - The United States and Philippine navies held a joint naval patrol on Saturday in dangerous southern Philippine waters, amid rising international concern about Islamist militancy and piracy in the region.

US Navy littoral combat ship USS Coronado joined a Philippine Navy frigate, BRP Alcaraz, in patrolling the Sulu Sea where numerous pirate attacks on commercial shipping have been made since 2015.

“Our at-sea operations with the Philippine Navy demonstrate our commitment to the alliance and deter piracy and illegal activities,” US Rear Admiral Don Gabrielson said in a statement issued by the US embassy in Manila.

There are international fears fighters sympathetic to Islamic State will cross maritime borders between Malaysia and Indonesia to join Muslim rebels who seized Marawi City in the southern Philippines five weeks ago.

About 300 militants, 82 security forces and 44 civilians have been killed in the fighting.

The naval patrols were held at the invitation of the Philippine government, the US embassy said. Philippine President Rodrigo Duterte does not allow joint patrols with the United States in the disputed South China Sea to avoid damaging its relations with China, which claims the sea as its own.

But he welcomes cooperation in the south due to increased militant activity. Two weeks ago, Indonesia, Malaysia and the Philippines held joint naval patrols in southern waters.