Oil companies should urgently intensify their efforts to fight climate change, the United Nations climate chief said in a letter replying to six European firms.

"I would call on you to devote urgent attention to scaling this action up and look forward to learning about progress later in the year," Christiana Figueres said in the letter, published on Thursday.

She also called on them to outline how they intended to meet the G7's call this month to decarbonisation their economies by the end of the century.

Europe's top oil and gas companies had written to Figueres, urging governments around the world to introduce a pricing system for carbon emissions.

The oil firms were BG Group, BP, Eni, Royal Dutch Shell, Statoil and France's Total. Figueres said she was confident the oil companies genuinely want to tackle climate change but told them "governments and civil society also need to be reassured of your sincere commitment."

By joining discussion over carbon pricing in particular, the oil companies should outline what prices they think are needed to facilitate switching to cleaner fuels and encouraging investment in technology to capture and store carbon emissions underground, she said.

"The world wants to be proud of your actions at this time of crisis and I look forward to your giving us every reason to be," she said.