The US State Department said Tuesday it is confident it will be vindicated in a probe into how it is handling a pending decision on whether to grant a permit for a massive US-Canada oil pipeline.

The agency's Office of Inspector General (OIG) revealed it is launching a "special review" of the department's handling of the Environmental Impact Statement (EIS) and National Interest Determination for the mammoth project.

A letter from US lawmakers, released by the Natural Resources Defense Council, had asked the OIG if TransCanada, the pipeline developer, improperly influenced the State Department's selection of a contractor for the EIS.

"We welcome this review," State Department spokeswoman Victoria Nuland told reporters, calling it "an opportunity for an impartial assessment" of how it has handled public consultations into whether or not to allow the pipeline.

"And we are confident that this assessment will bear out that we have conducted the Keystone pipeline review process consistently with existing US law and regulations," she said.

"And we will be cooperating fully with the Office of Inspector General," said Nuland.

The State Department has been holding public consultations on plans to build the 1,700-mile (2,700-kilometer) Keystone XL pipeline from the tar sands of Canada's Alberta province to the Gulf of Mexico in the southern United States.

In its long-awaited environmental impact statement on the project, the State Department said in August that the pipeline would be safer than most current oil transportation systems.

Many environmentalists fear a potential pipeline accident would spell disaster for aquifers in central US Great Plains states. That could disproportionately endanger rural towns and Native Americans, they say.

Friends of the Earth last month alleged that emails it obtained through the Freedom of Information Act showed that department employees held a "pro-pipeline bias and complicit relationships with industry executives."

The State Department denied the charges, saying it had behaved "transparently and evenhandedly" toward the pipeline project.

The State Department said last week it still aimed to issue a decision by year-end but cautioned the deadline could slip as the "first priority" is to ensure the pipeline's potential environmental impact is carefully studied.

Washington is considering the permit for the $7 billion project, due to stretch across 1,700 miles (2,700 kilometers), part of the broader $13 billion Keystone pipeline system.