The struggle for the world's remaining natural resources is becoming more murderous, according to a new report that reveals that environmental activists were killed at the rate of more than two a week in 2011.

The death toll of campaigners, community leaders and journalists involved in the protection of forests, rivers and land has risen dramatically in the past three years, said Global Witness.

Brazil – the host of the Rio+20 conference on sustainable development – has the worst record for danger in a decade that has seen the deaths of more than 737 defenders, said the briefing, which was released on the eve of the high-level segment of the Earth Summit.

The group called on the leaders at Rio to set up systems to monitor and counter the rising violence, which in many cases involves governments and foreign corporations, and to reduce the consumption pressures that are driving development into remote areas.

"This trend points to the increasingly fierce global battle for resources, and represents the sharpest of wake-up calls for delegates in Rio," said Billy Kyte, campaigner at Global Witness.