MANILA (Reuters) - Philippine troops killed about a dozen members of a pro-Islamic State group in air and ground assaults this week on the southern island of Mindanao, the army said on Wednesday, in what it called preemptive moves to thwart militant attacks on cities.

Helicopters fired rockets as army units moved to seize two camps of Bangsamoro Islamic Freedom Fighters (BIFF) rebels in marshlands in central Mindanao island in the south, said spokesman Lieutenant-Colonel Gerry Besana.

“This morning, two MG520 attack helicopters conducted air strikes to disrupt the BIFF’s plan to stage attacks,” Besana said.

The military wanted to send a message to militants that they should surrender, “or fight and be killed”, he said.

About a dozen guerrillas were killed in the assault that started on Monday, said Besana, citing intelligence reports and accounts from civilians who had fled the fighting.

The military has shifted its combat operations from Marawi, a battered lakeside town in Mindanao embroiled in a five-month conflict last year, to the island’s marshes where other pro-Islamic State militants operate.

Besana said the military had seized control of two rebel enclaves where guns and improvised explosive devices were being assembled. About 20 rifles and 100 kg of gun parts and bomb-making materials were recovered, he said.

More than 40 members of the BIFF, led by three radical Muslim clerics, have been killed since last month, when soldiers backed by helicopters and armored vehicles launched combat operations, Besana added.