MANILA, July 10 (Xinhua) -- Twelve terrorists have been killed in clashes with Philippine government security forces, bringing the death toll of Bangsamoro Islamic Freedom Fighters (BIFF) to 57 in the ongoing offensive in the southern Philippines that started on June 1.

The military said on Tuesday that eight were killed during a clash on Sunday in the villages of Shariff Saydona Mustapha town in Maguindanao province in the central Mindanao. Four others were killed on the same day in a separate clash in another part of town.

On Sunday, Brigadier General Cirilito Sobejana of the Army's 6th Infantry Division based in Maguindanao province, said the army has so far launched "sustained military operations" against the BIFF, an extremist group with links to Islamic State fighters.

There are up to 400 BIFF terrorists operating in central Mindanao, he said.

BIFF members are mostly disgruntled former members of the larger insurgent group Moro Islamic Liberation Front (MILF) which signed a peace accord with the government.

The BIFF, established in 2010 by former MILF commander Ameril Umbra Kato who died of a heart attack in 2015, is among the armed groups in Mindanao that pledged allegiance to the Islamic State of Iraq and Syria a few years ago.