The US State Department on Friday launched an environmental review of a new route proposed for the controversial Keystone XL pipeline that would transport oil from Canada's tar sands.

President Barack Obama rejected the initial proposal for the $7 billion pipeline early this year, saying he could not vouch for its safety in time for a deadline despite intense election-year pressure.

Republican presidential hopeful Mitt Romney lashed out at Obama's rejection, saying the incumbent fighting for a second term in November elections "demonstrates a lack of seriousness about bringing down unemployment, restoring economic growth and achieving energy independence."

House Republicans also seized on the issue and passed legislation in April mandating construction of the pipeline.

In February, TransCanada said it would go ahead with building part of the pipeline between Oklahoma and the Texas coast that does not require US presidential approval. It said work should begin this summer and take about a year.

TransCanada then submitted a new permit application in May for the northern portion of the pipeline with a revised route that would avoid Nebraska's environmentally sensitive Sand Hills.

The State Department issued a notice of intent Friday that it will prepare an environmental review of the new proposal which would extend from the border crossing at Phillips, Montana to Steele City, Nebraska.

It will also review the proposal to determine whether the pipeline will have an impact of historic preservation sites and has invited Indian tribes and other interested parties to comment.

Environmentalists fear an accident along the 1,700-mile (2,700-kilometer) pipeline would spell disaster for aquifers in central US Great Plains states. They also oppose the project because exploiting the oil sands requires energy that generate a large volume of greenhouse gases and say a change of the route will not lessen the pipeline's dangers.

"Keystone XL is a commitment to dirty fuels and pollution for decades," said Joe Mendelson, director of climate and energy policy at the National Wildlife Federation.