MANILA, Oct. 21 (Xinhua) -- Abu Sayyaf leader Isnilon Hapilon has been killed in an assault in the Philippine southern city of Marawi, Philippine Defense Secretary Delfin Lorenzana said on Saturday, citing the United States Federal Bureau of Investigation (FBI).

Hapilon, with alleged link to Islamic State (IS) extremists, is one of the most wanted terrorist leaders in the Philippines.

"We have received an official report that the U.S. FBI has confirmed that the DNA sample taken from a body recovered by our opening units in Marawi matches that of Isnilon Hapilon," said Lorenzana in a statement.

He added that "this process of verification is also being conducted on the cadavers of the other terrorists that have been recovered so far."

Lorenzana did not say how Hapilon's sample was sent to the FBI for DNA testing.

The Philippine military killed Hapilon and Omarkhayam Maute during an assault before dawn on Monday. Pictures of the slain militant leaders were distributed to the media a few hours later.

The U.S. State Department had offered 5 million U.S. dollars for the arrest of Hapilon.

Hapilon is the leader of the Abu Sayyaf Group, a jihadist organization in the Philippines founded with funds from al-Qaeda that also pledged loyalty to IS. The militant group is behind the spate of kidnappings and numerous bombing attacks in the Philippines.

Hapilon, along with the Maute brothers, led the May 23 attack on Marawi. The military said IS militants from neighboring Malaysia and Indonesia also took part in the siege.

Efforts to completely retake the remaining areas where more than a dozen of pro-IS militants have holed up continues, according to Armed Forces of the Philippines spokesman Restituto Padilla.

More than 1,000 people have been killed in the ongoing war that broke out on May 23, prompting Philippine President Rodrigo Duterte to place Mindanao under Martial law.

On Tuesday, Duterte declared to have liberated Marawi from the IS militants.