Soldiers walk past the body of a man slumped beside a tricycle following an armed attack in front of the temporary headquarters of the army, in Jolo on Friday. AFP

The authorities said Abu Sayyaf militants could be behind the midday blast, which also left nine other soldiers wounded on the island of Jolo, a stronghold of Daesh-linked gunmen.

"We're not discounting the possibility that it's the handiwork of Abu Sayyaf Group," regional military spokesman Major Arvin Arcinas told reporters after the attack on the special counter-terrorism unit's temporary headquarters.

The Philippines has renewed its campaign against the militants on Jolo this year after at least one suspected suicide bomber attacked the island's Roman Catholic cathedral in January, killing 21 people.

Friday's blast blew the roof off the sentry gate of the military camp and blackened its concrete walls, according to photographs of the aftermath of the attack on local television.

Three members of the military unit were killed and nine others were wounded, while two civilians — a motor tricycle driver and a woman street vendor — also died in the attack, army spokesman Colonel Ramon Zagala told AFP.

"This attack is meant to disrupt the intensified security operations and our operational tempo following series of recent operational gains in the area," Zagala said in a separate statement.

An AFP reporter on the scene saw a blood-soaked man slumped beside a motor tricycle on a street also stained with blood in front of the temporary headquarters of the 1,500-member First Brigade Combat Team.

The authorities could not say what kind of explosives were used.

Jolo and other remote areas of the southern Philippines are home to numerous armed groups including the Abu Sayyaf, which is notorious for kidnappings and bombings.

The militants, who demand large ransoms and have beheaded several Western hostages, have pledged allegiance to the Islamic State group.

The army deployed the specially trained unit in the Jolo town of Indanan four weeks ago.

In the same week, a Dutch birdwatcher held for years by the Abu Sayyaf was killed on Jolo during a firefight between his kidnappers and soldiers sent to rescue him.

Agence France-Presse