The European Union's top climate official called on Wednesday for a major overhaul of the UN's carbon credit mechanism amid concerns from environmental groups.

The Clean Development Mechanism "has been successful in some aspects but has also given rise to criticism, for example, with regard to environmental integrity," European Climate Action Commissioner Connie Hedegaard said.

"As a first step towards a more advanced carbon market the CDM therefore needs a major overhaul," Hedegaard said in a statement.

The commissioner said she would propose new restrictions on the use of credits from industrial gas projects under the EU's emission trading scheme after 2012, the end of its current trading period.

Under the CDM, companies in rich countries can claim carbon credits to offset against their own greenhouse-gas emissions if their project qualifies as an initiative that reduces emissions in a poor country. The credits can then be bought and sold, like any asset.

But environmental groups have asked for a revision of the mechanism, arguing that it lacks balance, has actually led to growth in the potent HFC-23 greenhouse gas and subsidised the chemical industry.

"The international debate has made it quite clear what changes to the CDM are needed ... and also what successor mechanisms should be put in place to make the carbon market an even more powerful instrument to reduce emissions," Hedegaard said.