Republican politicians in the US are fighting President Barack Obama's decision to reject a controversial pipeline project, in a move the White House says raises "serious" legal questions.

Republicans are trying to strip the president of his authority to decide the fate of the $7bn Keystone XL oil pipeline, which would carry tar-sands oil from Canada through six US states to refineries in the Gulf of Mexico.

They say Obama made the decision based on political considerations, and that the project would create thousands of jobs. Democrats, however, argue that a full environmental assessment must be carried out before the deal can be approved.

However, Kerri-Ann Jones, the US assistant secretary of state, says the proposed bill "imposes narrow time constraints and creates automatic mandates that prevent an informed decision".

Appearing before congress on Wednesday, Jones said: "The legislation raises serious questions about existing legal authority and appears to override foreign policy and national security considerations,"

The bill, sponsored by Lee Terry, a Nebraska Republican House member, would transfer authority over the 2,700km pipeline to the Federal Energy Regulatory Commission.

Obama had rejected the pipeline deal on January 18 "in the national interest", citing environmental and other concerns.

The Obama administration "says it had no choice [but to reject the deal] after Republicans forced a decision before a full study of Keystone could be completed", Al Jazeera's Kimberly Halkett reported from in Washington, DC.

"Congressional Republicans, also facing re-election [as Obama is], are vowing to press on with efforts to revive the project.

"They insist in the stagnant US economy, any environmental concerns are outweighed by the large number of jobs they say the Keystone pipeline will create."