A Philippine lawyer who specialised in investigating crimes against the environment has been ambushed and shot dead, police said Friday.

The murder on Wednesday of Mia Manuelita Mascarinas-Green deepened concerns that the Philippines is one of the world's most dangerous places for environmental campaigners, with more than 100 killed over the past 15 years.

Four motorcycle-riding gunmen opened fire after surrounding a van being driven by Mascarinas-Green -- with her children and nanny in the vehicle -- near her home on the central island of Bohol, the authorities said.

Mascarinas-Green was pronounced dead at a hospital but her children were unharmed, regional police spokesman Senior Inspector Reslin Abella told AFP.

"The victim is a known environmental lawyer. Investigators are checking whether the attack had any link to the cases she had handled in relation to environmental issues," Abella told AFP by telephone.

"They now have the identity of at least one of the perpetrators and a hot pursuit operation is ongoing," she said without naming the suspect.

Abella said police were at the moment unaware if Mascarinas-Green had been threatened previously in relation to her work.

Her children are twins, aged two, and a 10-year-old daughter, according to local media reports.

Her death brings to 112 the number of environmental campaigners murdered in the Philippines over the past 15 years, according to Filipino environment monitor Kalikasan.

This includes 12 since President Rodrigo Duterte took office seven months ago, Kalikasan said.

"Most of these cases remain unresolved as the government continues to ignore the threat against environmental defenders," Clemente Bautista, its national coordinator told AFP.

"What this means is that the perpetrators are emboldened to do it again and again because no one ever gets caught."

Greenpeace Southeast Asia executive director Yeb Sano also said the killing highlighted the culture of impunity in the Philippines, where powerful figures abuse a corrupt political and justice system to literally get away with murder.

"Those who cause environmental destruction are resorting to savage measures and deplorable acts to stop communities and people who are standing up to protect our imperilled environment," Sano said.

Environmental monitor Global Witness separately lists 88 killings of environmental activists and workers in the Philippines between 2010 and 2015.

The attacks spiked with 33 dead in 2015, ranking the Philippines as the second most dangerous country in the world for environmental campaigners behind Brazil.