The Philippine government has voiced outrage at "ruthless" illegal loggers intent on defying a nationwide ban on destroying forests, following the New Year's Day murder of an environment officer.

Two unidentified men armed with M-16 rifles shot environment department officer Alfredo Almueda in the head as he waited at a forest checkpoint to intercept a truck carrying logs on Tuesday, the government said in a statement.

"We condemn in the strongest possible terms the killing of yet another environment officer. We are angered and outraged by the attack," Environment Secretary Ramon Paje said in the statement issued late Wednesday.

Paje said Almueda, 59, was the victim of persistent efforts by illegal loggers to silence "environmental defenders".

"This attack clearly demonstrates the ruthlessness of those responsible for the rape and destruction of our forests," Paje said.

Almueda's killing brings to 21 the number of environment department personnel, deputised workers and volunteers who have been killed since a nationwide logging ban was imposed two years ago, according to government data.

The Philippines, a tropical Southeast Asian archipelago, has lost more than half its forest cover over the past century with only about 7.6 million hectares (18.8 million acres) left, Paje told reporters last year.

These forests are guarded by underfunded, outnumbered and poorly armed government personnel who often face resistance from illegal loggers often linked to powerful businessmen and corrupt officials.

Clemente Bautista, national coordinator for the non-government environmental group Kalikasan (Nature), said illegal loggers felt they could operate with impunity.

"Initially, they will be intimidated and if that does not work, (the loggers) file a case against the officials or have them removed from their post. If the officials are hard-headed they will even kill them," he said.

Out of all the 21 murders since the start of 2011, only one suspect for one of the cases has been caught, environment department assistant secretary Danny Nicer told AFP on Thursday.