A Canadian firm is offering new routes to build a multi-billion-dollar Canada-US oil pipeline, after its first proposal was rejected over environmental concerns, officials said Friday.

The US State Department said it was "committed to conducting a rigorous, transparent and thorough review" of the northern leg of the Keystone XL pipeline that would join an existing pipeline in Steele City, Nebraska.

It said the new application for a US permit from TransCanada Corporation "includes proposed new routes through the state of Nebraska," where environmental groups raised concerns about the pipeline's potential danger to a major aquifer.

But Friends of the Earth immediately criticized the new bid for the $7 billion pipeline that would stretch 1,700 miles (2,700 kilometers) from the tar sands of Alberta to Texas on the US Gulf Coast.

"The pipeline would still cut through the heart of Nebraska's fragile Sandhills and Ogallala Aquifer, which provides drinking water for two million people in the Midwest and supports $20 billion in agriculture," it said.

The environmentalist group had previously accused the State Department of conflicts of interests in conducting the pipeline review.

US President Barack Obama rejected the initial proposed 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."

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.

In announcing its new application for the link from the Canadian border with the US state of Montana to Nebraska, TransCanada said it "maintains its commitment to build Keystone XL as safely and reliably as possible."

The firm said it will adopt US federal regulatory conditions, including "a higher number of remotely controlled shut-off valves, increased pipeline inspections and pipe that is buried deeper in the ground."

The State Department said it will now hire "an independent third-party contractor" to help determine whether granting a permit for the proposal would meet US national interests.

Put in the balance are "energy security, health, environmental, cultural, economic and foreign policy concerns," it said in a statement.

"Previously when we announced review of alternate routes through Nebraska this past fall, our best estimate on when we would complete the national interest determination was the first quarter of 2013."

Russ Girling, TransCanada's president and chief executive officer, said the pipeline will reduce US "dependence on foreign oil and support job growth by putting thousands of Americans to work," according to a company statement.

"Keystone XL will transport US crude oil from the very large Bakken supply basin in Montana and North Dakota, along with Canadian oil, to US refineries," Girling said.