A US appellate court on Friday ordered the immediate release of an elderly nun and two other peace activists a week after overturning their sabotage convictions for breaking into a Tennessee nuclear defense facility in 2012, their lawyer said.

In a 2-1 decision last Friday, the 6th U.S. Circuit Court of Appeals reversed sabotage convictions against Megan Rice, 85, and US Army veterans, Michael Walli, 66, and Greg Boertje-Obed, 59.

The panel majority found that the three lacked the necessary intent for a violation of the federal Sabotage Act for the break-in at the Y-12 National Security Complex in Oak Ridge, an incident that embarrassed US officials and prompted security changes.

Attorney Bill Quigley said their legal team asked the court on Thursday for their immediate release, arguing that the two years they had already spent in prison was longer than what they would be sentenced for on the remaining charges.

Quigley, who is also a law professor at Loyola University in New Orleans, said the court ordered their immediate release on Friday until a formal resentencing.

"There wasn't any dispute that if they are not guilty of sabotage, they should be out of jail," Quigley said.

He added, however, that while federal prosecutors did not necessarily oppose their release, they still had time to appeal the court's reversal.

A representative for the US Attorney's Office in Knoxville, Tennessee, was not immediately available for comment.

The court upheld their convictions for the less serious crimes of injury to government property.

The three were convicted of cutting fences to get into the facility on the night of July 28, 2012, and they admitted to spray painting peace slogans and hanging banners. When a guard confronted them, they offered him food and began singing.

Prosecutors had contended the break-in at the primary US site for processing and storage of enriched uranium disrupted operations, endangered US national security and caused physical damage.

The three were convicted by a jury in May 2013: Rice was sentenced to three years and the two others to five years.

At the time of her sentencing in February 2014, Rice had asked the judge not to take her age into consideration and that to spend the rest of her life in prison would be the "greatest honor."