MANILA - Neutralizing Islamic militant group the Abu Sayyaf, which recently beheaded two foreign hostages and kidnapped seven more, is the top military priority of Philippine president-elect Rodrigo Duterte, his incoming defense secretary said Wednesday.

Delfin Lorenzana, who takes the reins of the Department of National Defense (DND) on Thursday, said the kidnappings of the extremist group -- which is also blamed for the worst terror attacks in Philippine history -- were already affecting the country economically.

"Our first priority is the Abu Sayyaf because that is the order of our new president: address the Abu Sayyaf immediately so we can neutralize them," he told reporters.

The Abu Sayyaf has defied two decades of government efforts to crush them and attacks such as its recent abduction of seven Indonesian seamen, prompting the country to bar its ships from carrying coal to the Philippines, have impacted the economy.

Lorenzana said that after conferring with military commanders, he would implement a plan "in a couple of days" to defeat the militant group.

WATCH: Visaya ready to fight Abu Sayyaf '24/7'

The Abu Sayyaf is a loose network of a few hundred Islamic militants, formed in the 1990s with seed money from Osama bin Laden's Al-Qaeda network, that has earned millions of dollars from kidnappings-for-ransom.

They are based in the heavily-forested, southern islands of Basilan and Sulu where their mastery of the terrain and assistance from local Muslim communities have helped them evade government pursuit.

Although its leaders have pledged allegiance to Islamic State, analysts say they are mainly focused on lucrative kidnappings.

The Abu Sayyaf earlier this year beheaded two Canadian hostages who were kidnapped from a yachting resort in September. Another hostage, a Norwegian seized in the same raid, is still being held along with other foreign and local captives.

Lorenzana said the military would continue its operations to recover the hostages and capture the gunmen.

He added that their second priority would be assisting the police in a nationwide operation against crime. Duterte, who takes office on Thursday, won the May 9 presidential elections by promising a ruthless campaign against criminals, backed by the military.

"If they need more troops, more warm bodies, we will support them, depending on their request," he added.

Dureza: Duterte can solve Abu Sayyaf problem