The head of the UN's panel on climate change has stepped down amid claims that he sexually harassed a colleague at an NGO in India.

Rajenda Pachauri has chaired the UN Intergovernmental Panel on Climate Change since 2002, and in 2007 accepted a Nobel Peace Prize on its behalf.

The IPCC "needs strong leadership and dedication of time and full attention by the chair in the immediate future, which under the current circumstances I may be unable to provide", Pachauri wrote in a letter to UN Secretary General Ban Ki-moon, reports AP.

Earlier this week, he withdrew from a Nairobi meeting to attend to "issues demanding his attention in India".

In India, Pachauri is under investigation following claims that he sexually harassed a 29-year-old woman while they both worked at the The Energy Resources Institute, a lobbying organisation he leads in New Delhi.

He denies all allegations.

Pachauri, 74, has served as chairman of the panel for 13 years.

IPCC vice chairman Ismail El Gizouli will serve as the panel's chairman until a new permanent incumbent is chosen by vote in October, said the organisation.

Pachauri's second term as chairman was due to end in October, and he previously announced that he did not intend to run for a third. He is known to suffer cardiac problems.

In his resignation letter, Pachauri said he "would be available for help, support and advice to the entire IPCC in its future work in whatever manner I may be called on to provide".

The IPCC's findings on global warming form the main guide for action by almost 200 governments around the globe.