Bishop Pablo Virgilio David of Caloocan, the Philippines, has expressed dismay that he has been unable to protect his flock from "wolves".

He was speaking after masked men shot dead a 27-year-old widow who had been helping bereaved families whose loved ones had been killed in the country's drugs war.

Bishop David, whose flock consists of more than a million Catholics in Caloocan, the country's fourth city, said his diocese had become a "killing field".

"Today, in utter shame and frustration, I declare I have not been a good shepherd to my flock," he said at a Mass for a conference on new evangelisation in Manila.