HELENA, Mont. - A Montana judge must decide whether to once again block the proposed Keystone XL oil pipeline or side with U.S. government attorneys who want him to uphold President Donald Trump's permit to cross the U.S.- Canada border.

District Judge Brian Morris hears arguments today by environmental groups seeking to halt the 1,900-kilometre pipeline and by government attorneys who say the presidential permit isn't subject to environmental laws.

Last year, Morris blocked construction after ruling officials had not fully considered oil spills and other environmental effects.

Trump signed the new permit in March, prompting the plaintiffs to accuse the president of trying to get around the judge's previous order.

A separate lawsuit by Native American tribes alleges Trump's approval did not take into consideration the potential damage to cultural sites.