Archbishop Desmond Tutu is one of eight Nobel Peace Prize winners who have signed a letter asking Prime Minister Stephen Harper to do what he can to stop the growth of Alberta's oilsands.

The letter comes three weeks after several Peace Prize laureates wrote a letter to American President Barack Obama asking him to block the proposed Keystone XL Pipeline, which would increase oilsands exports to the United States.

"Just as we called on President Obama to reject the pipeline, we are calling on you to use your power to halt the expansion of the tarsands -- and ensure that Canada moves towards a clean energy future," the letter says.

The letter to Harper is signed by almost all the same people who signed the one to Obama, except for the Dalai Lama, who did not sign the Harper letter.

The letter to Obama came after a request from the U.S.-based environmental organization Natural Resources Defence Council.

The laureates point out that Harper has called climate change one of humanity's biggest challenges. They call on him to back his words with action by using federal powers to halt further expansion of the oilsands.

"It would be wrong for humanity to choose a path that drives hundreds of thousands of species to extinction," the letter says.

"It would be wrong for a rich minority of the world's inhabitants to create a problem like climate change and then refuse to do its fair share to fix it.

"And it would be wrong for this generation to make this planet uninhabitable when we know that our children and grandchildren will be forced to deal with the consequences."

The letter says Canada is well-placed to be a leader in fighting climate change. It praises Ontario legislation that aims to phase out high-carbon, coal-fired power generation, calling it "probably the single most effective piece of legislation promoting renewable energy in North America."

The letter was co-ordinated through the Nobel Women's Initiative, a group that brings together the six living female recipients of the prize.

Besides archbishop Tutu, the signatories include Mairead Maguire, Betty Williams, Adolfo Perez Esquivel, Rigoberta MenchDu Tum, Jose Ramos-Horta, Jody Williams and Shirin Ebadi.

They are renowned for their work in promoting peace and human rights in South Africa, East Timor, Northern Ireland, Argentina, Iran and Guatemala, as well as for causes such as banning land mines.