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MANILA (Reuters) - The father of the leaders of the pro-Islamic State Maute group that seized control of a southern Philippine town in May died while in government custody, authorities said on Sunday.

Cayamora Maute was taken to a hospital on Sunday afternoon after his blood pressure rose but he died along the way, the Philippines’ prison bureau said.

The May 23 occupation of Marawi City by the Maute group, led by his two sons and which has pledged allegiance to Islamic State, triggered a brutal urban battle with military forces that entered its fourth month last week.

It has raised concern that Islamic State, on a back foot in Syria and Iraq, is building a regional base on the Philippine island of Mindanao that could pose a threat to neighboring Indonesia, Malaysia and Singapore too.

More than 700 people, including 130 soldiers, have been killed since the militants, aided by foreign fighters from Indonesia, Malaysia and the Middle East, seized control of city of 200,000.

Maute had several ailments when he was taken into custody in June, including diabetes and hypertension, Xavier Solda, spokesman at the Bureau of Jail and Management and Penology told reporters.

The extent of his involvement in the group is not immediately clear but when he was arrested in June, a military spokesman expressed hope he could persuade his sons to stop fighting and surrender.

“This is an unfortunate incident for his family, but more so to the victims of terrorism in Marawi and their relatives who are awaiting justice and expecting that Cayamora would answer and atone for his involvement in the Marawi rebellion,” Armed Forces Chief of Staff General Eduaro Año said in a statement.

Philippine President Rodrigo Duterte has extended martial law on the southern island of Mindanao until the end of the year, to give him time to crush the rebel movement.