Gunmen in the southern Philippines ambushed a tribal chieftain who was campaigning against mining and destruction of forests, killing his 11-year-old son in the latest of several attacks on environmental activists, a human rights group said Wednesday.

Amnesty International said Lucenio Manda was driving his son, Jordan, to school in Zamboanga del Sur province on Tuesday when assailants fired at them. The elder Manda was wounded and said later via text message that his son was sacrificed to protect his people's rights and ancestral domain.

"It is very painful and I thirst for justice. I vow to continue my struggle in order not to make my son's death in vain," Manda said.

Police said they are investigating. Amnesty International noted that Manda's family has been targeted in the past. His cousin was fatally shot a decade ago.

Manda, the chief of the Subanen animist tribe in a region dominated by Christians and minority Muslims, has led efforts against logging and mining, said the head of Amnesty International Philippines, Aurora Parong.

Manda filed a court petition to revoke mining permits at the Pinukis Range Forest, among the last untouched forests in the resource-rich southern region where several multinational companies are extracting gold and silver.

"The killing of Jordan Manda, groomed to be a next Timuay (tribal leader), is a painful reminder that indigenous peoples are not protected," Parong said in a statement. She called on President Benigno Aquino III's government to bring perpetrators to justice and stop what the group says is a culture of impunity that has left 36 tribal activists dead over the last several years.

"The indigenous peoples' future depends on genuine efforts and concrete actions by the government to fulfill their duties in holding mining corporations accountable for any human rights abuses," Parong said.

New York-based Human Rights Watch in July said it has documented three cases since October 2011 in which critics of mining and energy projects have been killed in the Philippines, allegedly by paramilitary forces under military control. The military denied any involvement.

Amnesty International also quoted a police report saying that guards hired by a Canadian mining company were responsible for a fatal shooting in July.

That month, Aquino issued a presidential decree that encourages mining investment through streamlined policies and guidelines. However, human rights group say it does not address abuses against the local population.