MANILA, May 25 -- Six more soldiers and 13 members of the militant Maute Group in Marawi City were killed as armed clashes between the two sides continued on its third day on Thursday.

Thirteen of the Maute Group members perished as government troops cleared two bridges, said Capt. Jo-ann Petinglay, spokesperson of the military's Western Mindanao Command.

Petinglay said soldiers battled around 30 heavily-armed members of the group for about two hours.

"Thirteen more terrorists were killed after two hours of armed confrontation with the same troops. Three high powered firearms were also recovered from the encounter site," said the spokesperson.

This brings to 31 the number of Maute members killed since the fighting broke out Tuesday afternoon. Operations on Wednesday also resulted in the death of six soldiers and wounding of seven others.

Philippine President Rodrigo Duterte has imposed martial law on the whole island of Mindanao, including island provinces of Basilan, Sulu and Tawi, a few hours after militant group Maute attacked Marawi, a city in the southern Philippines on Tuesday.

Duterte has said he might expand the coverage of his martial law declaration to include the entire country if the violence in Marawi city spills over into the Visayas and Luzon islands to the north of Mindanao.