The US Forest Service has told a federal court it is not sure when it could approve a land swap allowing Rio Tinto to develop the Resolution Copper mine in Arizona, a surprising reversal that boosts several Native American groups opposed to the project.

The complex case involves a long-running conflict between a mining company hoping to supply more than a quarter of US copper demand for the green energy transition and Indigenous groups seeking to preserve Arizona's Oak Flat campground, a site of religious importance that would be destroyed by the mine's construction.

In 2014, the US Congress approved a land swap that required an environmental report to be published, which former President Donald Trump's administration did shortly before leaving office. President Joe Biden unpublished that report in March 2021 to give his administration time to review concerns from Native Americans, though he was not able to permanently block the mine.

The case has wound its way through several courts. Joan Pepin, an attorney for the Forest Service, told judges during a March hearing the report would be republished "this spring."

Rumors have swirled in recent weeks that the Biden administration was on the verge of re-publishing that report, and several Rio executives made plans to travel to Arizona next week.

Late Thursday night, though, Pepin sent a letter to the court saying government officials were still meeting with Native American tribes.

"The department has not yet identified a timeframe for completing its review," Pepin said.

Neither Pepin nor other US Forest Service officials could be reached for additional comment.

Previously, officials had said they would alert the court at least 60 days before re-publishing the report. Once re-published, officials would have an additional 60 days to transfer the land to Rio Tinto.

Rio Tinto said it would continue to try to talk with tribes and believes there is significant local support for the mine. "Our team is ready to advance the project in collaboration with Native American tribes, local communities, and labor," said Vicky Peacey, general manager of the Resolution project.

BHP Group , which is helping Rio develop the mine, declined to comment.

"It's quite unusual for the government to have to take back something their attorney told the court under questioning," said Luke Goodrich of Becket Law, a religious liberty legal group involved in the case.

Representatives for the San Carlos Apache tribe have vowed that if they lose, they will appeal the ruling to the US Supreme Court.

"There is overwhelming opposition in Indian Country to the Resolution mine and that will not change," said Terry Rambler, San Carlos Apache tribal chairman.

Local officials who support the project said they were angered by Pepin's letter.

"Every time there's another delay to this process means that investors in our community may decide this isn't the place to invest because the federal government can't make up its mind," said Mila Besich, the Democratic mayor of Superior, a town of 2 500 that abuts the mine site.