The whereabouts of the missing Swiss environmental activist, Bruno Manser, remain unknown, despite high-level diplomatic efforts to track him down.

Nothing has been heard from Manser since he disappeared in the Malaysian province of Sarawak last year.

The Swiss ambassador to Malaysia, Rudolf Staub, has concluded a three-day fact-finding mission to Sarawak, but has not been able to find any new evidence of what happened to Manser following his disappearance.

Staub had hoped his personal intervention would lead to new information. His discussions with Sarawak officials have been "constructive, but have offered no clue as to the whereabouts of Manser," the ambassador said in a statement.

Manser spent six years living with Borneo's Penan tribe and publicising their grievances. The Penans are one of the few remaining nomadic forest peoples, and campaigners say their way of life is threatened by logging.

Manser fell foul of the Malaysian authorities, which declared him "persona non grata" 11 years ago. In July last year, the Malaysian prime minister, Mahathir Mohammad, described the Swiss activist as a "fanatic" with a hidden agenda.

He was arrested and deported by the Malaysian authorities in the spring of 1999, and threatened with a fine and imprisonment if he returned.

swissinfo with agencies