There is a "catastrophic" gap between national pledges to reduce greenhouse gas emissions and the actions needed to cap global warming below two degrees Celsius, the UN's environment chief warned Tuesday, days ahead of global climate talks in Bonn.

Even if fulfilled, these voluntary pledges -- inscribed along with the 2 C target in the 2015 Paris climate pact -- would see the world heat up 3 C (5.6 F), unleashing deadly heatwaves, superstorms and rising seas, UN Environment said in its annual Emissions Gap report, the bleakest ever.

"Governments, the private sector and civil society must bridge this catastrophic climate gap," said Eric Solheim, head of the UN agency.