A Canadian firm has reapplied for a US permit to build a multi-billion dollar Canada-US oil pipeline, after its first proposal was rejected over environmental concerns, both sides said Friday.

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

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

On January 18, President Barack Obama rejected the proposed pipeline from the tar sands of Alberta to the US Gulf coast, saying he could not vouch for its safety by a deadline despite intense election-year pressure.

Obama's political rivals had given him 60 days to make a decision on whether to approve the $7 billion, 1,700-mile (2,700-kilometer) pipeline route, forcing him to choose between environmentalists and industry.

The Obama administration said TransCanada could resubmit the Keystone XL project but that officials were not able to assess its plan by a February 21 deadline put into law by the Republican majority in Congress.

Republican presidential hopeful Mitt Romney lashed out at Obama's rejection, saying he "demonstrates a lack of seriousness about bringing down unemployment, restoring economic growth and achieving energy independence."

TransCanada said in February 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."

It 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 assist" it in determining whether granting a permit for the proposal would meet the US national interest.

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

"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," it said.

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.

[US President Barack Obama speaks at the southern site of the Keystone XL pipeline in March 2012 via AFP / Tom Pennington]