More than a dozen companies, mainly from Saudi Arabia, are bidding for two million tonnes of carbon credits at an auction in the Kenyan capital, Nairobi.

Demand for carbon offsets is growing fast as companies seek to reach their target of net-zero emissions.

The organisers of the auction say almost three-quarters of the carbon credits are being generated by projects in Africa, including supplying efficient cooking stoves in Kenya and Rwanda.

Large companies looking to offset their own carbon emissions - including the airline Saudia - will be bidding in Nairobi.

Within the next decade, the global market for these voluntary carbon credits is expected to be worth tens of billions of dollars.

But critics have in the past questioned the quality of some so-called green projects set up in Africa.

There is also some concern that with an option to buy carbon credits, companies including airlines are not under enough pressure to cut their own emissions.