Police Chief Superintendent Billy Beltran confirmed the arrest Friday of the men near Zamboanga City Water District around 3.20 a.m. (0720GMT) and said that around two hours later an explosion had gone off near a military detachment in the east coast of the city but no one was reported injured.

Initial investigations also showed an unidentified person had lobbed a grenade at a police station at dawn, he said.

Spokesperson Superintendent Rogelio Alabata said that police operatives backed by a SWAT team had led to the arrest of the three suspects.

"They are believed to be from the Sulu-based Abu Sayyaf group. It is believed also that the arrested ASG members were among those who slipped from Sulu due to the intensive military operations," said Alabata.

Two fragmentation grenades, guns and cellphones were recovered from the suspects.

Police and military security forces have been on high alert since Duterte declared the indefinite state of lawlessness following the explosion at a night market at Roxas Street in Davao City, where Duterte previously served as mayor.

Duterte has vowed to crush the Abu Sayyaf, which has claimed responsibility for the blast and made further threats as it is under an intensified military offensive from the military in Sulu and Basilan in the south.

The navy has also deployed its largest vessel to lead a naval blockade against the militants, in case they try and escape via sea.

On Friday, a spokesperson for the Autonomous Region in Muslim Mindanao humanitarian outfit said that around 4,000 mostly Muslim families fleeing towns where troops are battling the Abu Sayyaf in Sulu had received help in a two-day humanitarian mission.

Myrna Jo Henry told Anadolu Agency that the team distributed relief goods and gave medical assistance to some families displaced in Patikul town, a bastion of the Abu Sayyaf.

Since 1991, the Abu Sayyaf -- armed with mostly improvised explosive devices, mortars and automatic rifles -- has carried out bombings, kidnappings, assassinations and extortion in a self-determined fight for an independent province in the Philippines.

It is one of two militant groups in the south to have pledged allegiance to Daesh, prompting fears during the stalling of a peace process between the government and the Moro Islamic Liberation Front that it could make inroads in a region torn by decades of armed conflict.