Ernesto Abella made the statement as government security troops continued to battle Islamist militants who have pledged allegiance to the Islamic State in Marawi.
The militants, waving the black IS flag, have occupied parts of the war-torn city since May 23, prompting Duterte to declare martial law in Mindanao.
“The request of the president for additional 20,000 troops is part of our intensified security posture to guard areas in the country where there are continuing security threats,” Abella said.
“The deployment of troops to Marawi and other points in Mindanao needs to be rebalanced to ensure maximum effectiveness,” he added.
The Philippine military numbers about 125,000 and faces numerous threats apart from the IS-inspired militants in Mindanao, such as communist guerrillas scattered all over the archipelago and territorial disputes with China in the South China Sea.
In a trip to Marawi last week, Duterte asked Congress for the funding for 20,000 additional troops but did not specify how much he needed.
Military spokesman Colonel Edgard Arevalo said Sunday the defeat of the fighters in Marawi was coming soon but he would not give specifics.
“We are closer there now than before. We can say that we are really at the culminating part of our operations in Marawi,” he said.
