Islamist militants who seized the Philippine town of Marawi two weeks ago have stockpiled weapons and food in mosques, tunnels and basements to prepare for a long siege, officials said on Monday.

Among the several hundred militants linked to the Islamic State group are fighters from Indonesia, Malaysia, India, Saudi Arabia, Chechnya and Morocco.

The battle for Marawi has raised concerns that the ultra-radical jihadi group is building a regional base on the island of Mindanao, at the southern end of the Philippines.