President Rodrigo Duterte of the Philippines has asked Congress to extend martial law in the south of the country until the end of the year.

He said the rebellion there will not be defeated by Saturday, the end of his original 60-day martial law proclamation.

Mr Duterte declared martial law on May 23 following a bloody siege of the southern city of Marawi by Islamic State group-aligned militants, the most serious security crisis he has faced since assuming power in June last year.

In a letter to Congress on Tuesday, Mr Duterte said after consulting security officials, he concluded that it will take more time to quell the rebellion.

He asked Congress to extend martial law until December 31.

Under the constitution, the president can declare martial law for 60 days, and can ask Congress to extend such a proclamation to a period of time to be determined by the politicians.

More than 550 people, including 411 militants, have been killed in nearly two months of fighting in Marawi, a bastion of Islamic faith in the south.