Philippine President Rodrigo Duterte has asked Congress to extend martial law in the southern island of Mindanao until the end of the year to crush rebels inspired by the Islamic State militant group, his spokesman said Tuesday.

Mindanao, an island of 22 million people with a history of separatist and Marxist rebellion, was placed under military rule on May 23 after rebels from the Maute and Abu Sayyaf groups took over parts of the city of Marawi.

The insurgents have put up fierce resistance, with scores of fighters still holed up in Marawi's commercial heart, enduring seven weeks of ground offensives, airstrikes and artillery bombardments to prolong a crisis that has killed more than 500 people and displaced at least 260,000.