The state of Nevada has displayed "remarkable" disregard for national security in its continued legal battle over the secret shipment of weapons-grade plutonium to a site near Las Vegas, according to the U.S. Department of Energy.

The department has urged a federal judge to dismiss the state's lawsuit challenging the shipment of the highly radioactive material while a U.S. appellate court considers the state's appeal of an earlier ruling refusing to temporarily ban any future shipments.

Nevada says the Trump administration abused "top secret" classifications to meet the court-ordered removal of plutonium from South Carolina by the end of this year.

Energy Secretary Rick Perry and Sen. Catherine Cortez Masto, D-Nev., have released a letter he sent pledging to expedite removal of the plutonium beginning in 2021 and promising no more will be sent there.

The state's lawyers said in court documents that Perry's department cannot be trusted because of past misrepresentations about the covert shipment. The state said the Energy Department has selectively declassified information as it suited DOE's needs.

The department would not disclose the type of plutonium coming to Nevada, the form in which it would be transferred, exactly how it would be packaged, when it would arrive, the route that it would take, how long it would stay and many other crucial details that Nevada's public officials and first responders need to know, the state's lawyers said.

The Energy Department insists the plutonium was properly classified for security. It first disclosed the shipment Jan. 30, the same day Judge Miranda Du denied the state's request for an injunction that would temporarily halt all shipments.

"It should not be surprising that the United States carefully secures this information and it is remarkable that Nevada finds fault with the result," the agency's lawyers wrote.

The 9th U.S. Circuit Court of Appeals plans to hear arguments on Nevada's appeal in August, including its request to order removal of the plutonium until the appellate court decides whether the original shipment was legal.

Nevada Attorney General Aaron Ford's spokeswoman, Monica Moazez, said Wednesday that Perry's commitment not to send more plutonium to the state "marks a promising step" but added that the state's lawyers will continue in court to "permanently ensure this shipment of plutonium is timely and safely removed, and prevent future shipments."

Nevada's lawyers also want the judge to allow them to gather additional evidence based on the allegations the DOE acted in bad faith by arguing in court repeatedly that it didn't have to disclose when the shipment would occur due to national security concerns before acknowledging it had been trucked to Nevada months before. They say the department is resisting in order "to avoid damaging and inevitably embarrassing facts."

The federal government is under a court order in South Carolina to remove a ton of weapons grade plutonium from the Savannah River site there by Jan. 1, 2020.

Nevada's lawsuit filed in November said the agency illegally determined last summer it could transport the material from South Carolina to Nevada without completing a full-blown environmental impact statement under the National Environmental Policy Act to assess potential health and safety dangers.

Nevada's case relies partly on the DOE's past arguments in South Carolina that it was impossible to prepare such an assessment so quickly.

But when a judge ordered removal of the material anyway the department completed a less-detailed supplemental analysis in August that concluded a full-blown environmental impact statement wasn't necessary because it intended to ship the material in its current weapons-grade form instead of diluting it into waste.

"DOE's predetermined solution was to slap a new label on one ton of South Carolina's plutonium and call it 'weapons grade' so DOE could try to shoehorn it into prior environmental analyses," Nevada's lawyers wrote. The agency "worked backward to justify its selection and hurried to ship before Nevada found out."

The DOE acknowledged it originally planned to convert defense plutonium into waste for disposal, but said in the new documents it abandoned that plan when forced to comply with the court order.