PHNOM PENH (REUTERS) - The United States is ending a Cambodian aid programme aimed at protecting one its biggest wildlife sanctuaries, citing worsening deforestation and harassment of those who speak out about destruction of natural resources.

The US embassy in Phnom Penh said it had invested more than US$100 million (S$133.67 million) on combating deforestation and despite some progress, high rates of illegal logging had continued The Prey Lang Wildlife Sanctuary had since 2016 lost approximately 38,000 hectares of forest, or nearly 9 per cent of its forest cover, the embassy said.

It said Cambodian authorities were not adequately prosecuting wildlife crimes or stopping illicit activities.

"In addition, the government continues to silence and target local communities and their civil society partners who are justifiably concerned about the loss of their natural resources," the statement said.

The embassy said the aid would be redirected to support civil society, private sector and local conservation efforts.

In February, authorities detained and later released environmental activists protesting inside the sanctuary.

The Cambodian Centre for Human Rights (CCHR) said four environmental activists were arrested on Thursday alone.

The embassy said it would continue to engage with the government on climate change and environmental issues of mutual and global concern.

The government denied large-scale illegal logging activities were continuing at Prey Lang but said the withdrawal of the US aid showed Cambodia was capable of protecting its own environment .

"Large-scale natural resource crimes in the Prey Lang Wildlife Sanctuary and other protected areas no longer occur, but small-scale crimes continue to occur," said environment ministry spokesman Neth Pheaktra.