Twelve soldiers have been killed in clashes with the Abu Sayyaf armed group on a remote southern island in the Philippines, as a government offensive against the fighters entered a fifth day.

Philippines army Major Filemon Tan said on Monday that five soldiers were also wounded in the firefight that lasted more than an hour in the jungles of Patikul town on Jolo island.

"The fighting was really intense. We lost 12 men," Tan said, according to Reuters news agency.

"You can really expect heavy casualties from both sides due to [the] volume of fire," he continued, adding that it was unclear how many fighters from the armed group were killed but he estimated more than 30.

Already, more than 20 Abu Sayyaf fighters have been killed since Thursday when the military launched an air-and-ground offensive in Patikul, an Abu Sayyaf stronghold, after newly elected Philippines President Rodrigo Duterte, who started his six-year term on June 30, ordered troops to "destroy" the group.

READ MORE: Philippine troops kill more than 40 Abu Sayyaf fighters

Abu Sayyaf, known for kidnapping for ransom and beheading many of their captives, has fought on through successive Philippine governments. The group has entrenched its network with vast sums of ransom money in what has become one of Asia's most lucrative kidnapping rackets.

The group, whose name translates as "Bearer of the Sword", has pledged allegiance to the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), and was previously allied with al-Qaeda.

In June, Abu Sayyaf claimed they had beheaded Robert Hall, the second Canadian man who had been held captive by the group since September 2015. Another Canadian hostage was beheaded in April, after a ransom was not paid.

Last week, two Indonesians said they escaped captivity but there was speculation the Abu Sayyaf freed the pair after their families paid their ransoms.

Duterte's new defence minister recently said the elimination of Abu Sayyaf was his top security priority.

Eight Indonesians, eight Filipinos, five Malaysians, a Dutch and a Norwegian national are still being held by the group.

It was not known if the hostages were in Patikul jungle when the military assault began on Thursday.