South Africa's peace icon Desmond Tutu on Friday urged the United States to reject Canada's Keystone XL pipeline, saying the "carbon bomb" would upend the US role fighting climate change. The Nobel Prize-winning archbishop who fought apartheid led a letter to Secretary of State John Kerry urging him to reject the 1,179-mile (1,897-kilometer) pipeline, which would bring oil from the carbon-intense Alberta tar sands into the United States. "The verdict on whether to approve or reject the Keystone XL pipeline could, in just one stroke, confirm or condemn America's prospects for climate leadership," said the letter, released by the advocacy group Avaaz. "This is a US policy decision that will have truly global significance. Keystone has been called the 'fuse to the biggest carbon bomb on the planet,'" said the letter, timed for the final day of a public comment period on the pipeline. Other signatories to the letter included Yeb Sano, the Philippine diplomat who threw a spotlight on environmental dangers by fasting during UN-led climate negotiations last year, along with Australian Greens party leader Christine Milne and Britain's sole Green member of parliament Caroline Lucas. The letter reminded Kerry -- who ultimately makes the decision on the pipeline due to its international nature -- of his past calls for greater international action on climate change, including in a speech last month in Jakarta. Kerry has said only that he is making a "very intensive evaluation" of the proposal. A State Department report last month said that the pipeline would not substantially impact climate change as Canada would likely extract the oil anyway. A government watchdog later rejected a challenge by environmental groups who said that the third-party contractor which wrote the study had conflicts of interest. Canada's conservative Prime Minister Stephen Harper and US Republicans have pushed hard for approval of the pipeline, arguing that it would create jobs and increase US energy security by decreasing the need for Middle Eastern oil. House Speaker John Boehner has called for the United States to expand its energy supply, including through the Keystone pipeline, as a way to counter President Vladimir Putin of Russia, a major gas exporter.