A judge has stopped the transport of giant oilfield equipment across scenic roads in Montana, in a victory for campaigners against Alberta's tar sands.

The injunction granted by a district judge in Missoula bars Imperial Oil, a Canadian subsidiary of ExxonMobil, from going ahead with their plans to transport the over-sized mining equipment across the state.

Tuesday's decision delivers a new setback to the efforts of Alberta tar sands producers to expand their production and exports to the US.

The oil companies had originally planned to transport 207 pieces of the equipment, which is made in South Korea, from a port in Washington state across Idaho and Montana to the Alberta tar sands.

But a district court judge on Tuesday agreed with the National Wildlife Federation and other conservation groups that the oil companies and the Department of Transportation had not obtained the necessary environmental clearances to move the over-sized equipment along the scenic routes.

In his decision, Dayton also criticised state authorities for failing to conduct their own independent oversight of the transports but delegating the job to a contractor paid by the oil companies.

Dayton did not rescind permits already granted, and he dismissed the concerns from local landowners about the noise and dust generated by the shipments.

But he warned the oil companies could not go ahead without further environmental review.

The Obama administration is in the final stages of signing off on a new 1,600-mile pipeline project to transport oil from the tar sands across the American heartland to Texas oil refineries.

Oil producers have been fighting against demands for additional environmental oversight of the project.

But that position has grown increasingly precarious in the face of growing grassroots opposition and an unrelated pipeline accident over the Yellowstone river earlier this month.

A spokeswoman for Exxon admitted this week that tar sands oil – thicker and more corrosive than crude – was in the same pipeline. But she said it was not in an area affected by the leak.