Manilla – Around 400,000 people have been displaced in the ongoing clashes between Philippine forces and extremists in the city of Marawi, officials announced on Saturday as they warned of the possible spread of diseases among the people.

Marawi has been turned into a ghost town after it was overrun by ISIS-linked extremists on May 23.

For over a month, the army has launched a major campaign to regain control of the city, employing helicopters and armored vehicles to that end.

At least 400 people have been killed in the unrest and the fighters are still holed up in some neighborhoods of the city. They have resorted to sniper fire and explosives to slow the army advance.

On Sunday, the Philippines and Indonesia will jointly patrol the Celebes Sea this week to stop militants from reaching the Philippines’ restive southern island of Mindanao, where Marawi is located, an army spokesman said on Sunday.

The joint patrol is aimed at strengthening border security and improving interoperability, military spokesman Major Ezra Balagtey said in a statement. The two countries’ warships will sail from the Philippines’ Davao city on Thursday.

“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,” said Balagtey.

Regional governments fear fighters sympathetic to the ISIS will cross maritime borders from Malaysia and Indonesia to join rebels who seized Marawi five weeks ago.

About 300 militants, 82 members of the Philippines’ security force and 44 civilians have been killed in fighting.

The coordinated patrol is the third in the region in a month as pirates and militants step up attacks on commercial shipping.