The United States said Wednesday it may fail to decide on whether to issue a permit for a proposed multi-billion dollar oil pipeline stretching from Canada to Texas by the end of 2011 as planned.

State Department spokeswoman Victoria Nuland said the "first priority" is to ensure the pipeline's potential environmental impact is carefully studied rather than to meet the year-end goal set by US Secretary of State Hillary Clinton.

"The secretary's goal remains, the department's goal remains, to complete the process before the end of the year so a decision can be made before the end of the year," Nuland told reporters.

"But, obviously, our first obligation to the American people, to the president, is to ensure that we do this in a rigorous, transparent and thorough way," she added.

"We'd like to get it done by the end of the year, but if thoroughness demands a little bit more time, nobody's slammed the door on that," Nuland said when pressed on whether the deadline could slip.

Washington has launched consultations on the 1,700-mile (2,700-kilometer) Keystone XL pipeline which would run from the tar sands of the Canadian province of Alberta to the Gulf of Mexico in the southern United States.

The Keystone XL pipeline proposed by TransCanada would begin in Alberta in western Canada and pass through the US states of Montana, South Dakota, Nebraska, Kansas and Oklahoma before ending up at refineries in Texas.

A number of environmental and citizen groups are fighting the pipeline because exploiting the unconventional oil sands of Alberta requires energy that produces a large volume of greenhouse gasses.

The US State Department is handling public consultations as the pipeline would run across the border with Canada.