Some of the world's best-known Nobel Peace Prize winners are urging U.S. President Barack Obama to reject a proposal to extend the Keystone XL pipeline.

The Dalai Lama, South Africa's Archbishop Desmond Tutu, and Iran's Shirin Ebadi are among nine Nobel laureates who signed a letter to Obama saying the oil that would flow through the pipeline is dirty, toxic and corrosive.

The TransCanada pipeline would carry oil from Alberta's oilsands through six states. The State Department is now looking at whether the extension would be in America's national interest and Secretary of State Hillary Clinton is expected to make a decision on the extension by the end of the year.

The laureates say they fear a pipeline leak would contaminate the Ogallala Aquifer, one of the world's largest fresh water aquifers, arguing the existing pipeline, which has been operating for one year, has leaked 14 times.

Natural Resources Minister Joe Oliver said Wednesday that Keystone's environmental risks are less than those of other domestic pipelines and the National Energy Board was "positive" about its economic impact. Oliver also pointed to a comment last week by U.S. Energy Secretary Stephen Chu that the pipeline was likely to be approved.

"So we are increasingly optimistic about the likelihood of a presidential permit, which will be based on [Chu's] analysis of the national interest," Oliver said.

"I'm not taking anything for granted. We respect of course the decision-making process in the United States.  It's up to the secretary of state to look at this project."

Shawn Howard, a spokesman for TransCanada, says the Keystone extension will be the safest pipeline ever built in North America. He says the company has agreed to add an extra 57 safety mechanisms, like more shutoff valves through sensitive environmental areas, after feedback from U.S. agencies.

"While we respect that people will have different opinions, we would think that in the case of these Nobel Peace Prize winners that they would be particularly interested to know that the oil that will flow through the entire Keystone system is conflict-free oil. You can't say that about other regions of the world where oil is produced, where it's fought over," Howard said.

The letter follows two weeks of protests in Washington, D.C.

Mairead Maguire and Betty Williams of Ireland, who shared the prize in 1976, Adolfo Perez Esquivel of Argentina, who won in 1980, Rigoberta Menchu Tum of Guatemala, 1992 winner, Jose Ramos-Horta of East Timor, who took the prize in 1996, and Jody Williams of the United States, who won in 1997, also signed the letter.