A U.S. judge says environmentalists and Native Americans can proceed with lawsuits challenging President Donald Trump's approval of the Keystone XL oil pipeline from Canada.

In a Friday ruling, U.S. District Judge Brian Morris expressed skepticism over government arguments that Trump has unilateral authority to approve the $8 billion pipeline.

But Morris denied a request for a court injunction to prohibit preliminary work on the pipeline, since no such work is planned until spring 2020.

Morris had blocked the project last year, prompting Trump to issue a new permit in March in an attempt to circumvent the courts.