The Moro Islamic Liberation Front is seeking to prevent the militants from gaining a foothold in an area of Mindanao island.
The Moro Islamic Liberation Front is seeking to prevent the militants from gaining a foothold in an area of Mindanao island that the MILF calls its own, senior rebel leader Mohagher Iqbal told AFP.
Iqbal said 50 or so militants had pledged allegiance to IS and had ties to gunmen waging a deadly three-month-old battle with government forces in the southern city of Marawi.
"The radical elements are growing as a force," Iqbal told AFP by telephone.
An AFP video journalist who joined the MILF soldiers on Tuesday said the fighting was centred on vast marshy forests and farmlands near the town of Datu Salibo.
MILF men were filmed firing automatic weapons from a house, crossing streams and marshes with their machine guns, automatic rifles and rations raised above their heads, and inspecting the ruins of burnt houses.
Datu Salibo is about 100 kilometres (60 miles) south of Marawi and about 800 kilometres south of Manila.
Iqbal put the MILF death toll from the fighting, which began early this month, at 10.
He said the militants had also sustained fatalities, although he could not give confirmed numbers.
The 10,000-strong MILF has been leading a decades-long rebellion to establish an independent or autonomous homeland in Mindanao for the mainly Catholic Philippines' Muslim minority.
The conflict has claimed more than 100,000 lives.
The MILF signed a peace agreement with the previous administration in 2014 that envisages an autonomous Muslim homeland in Mindanao, and is hoping President Rodrigo Duterte can shepherd a final deal through a reluctant Congress.
However a range of more hardline militants, including former MILF members, oppose any form of peace with the government and some have in recent years pledged allegiance to IS.
The MILF has repeatedly warned that, if the peace process collapses, it will lose many of its younger members to IS-aligned groups.
Hundreds of IS-supporters occupied parts of Marawi in late May.
Government forces have been unable to dislodge the militants in Marawi despite a massive, US-backed military campaign.
The conflict has claimed more than 700 lives.
Iqbal said the MILF's current battles against another group near Datu Salibo showed IS was becoming a threat in other parts of the south.
"They're an affiliate of ISIS and they plant the black (ISIS) flag wherever they go," Iqbal said, using an alternate acronym meaning Islamic State of Iraq and Syria.
"They cause trouble everywhere. We are against their way of life. We are against terrorism."