(CNN) A Russian court on Tuesday remanded the case of Trevor Reed, a former US Marine detained in Russia, to a lower court for review, said US Ambassador John Sullivan, who was at the proceedings.
Sullivan is "very disappointed that justice has again been denied," he said.
"Unfortunately, the justice that Trevor deserves has been denied ... His appeal was not decided today, the proceedings continue, and Trevor remains in prison for a crime he didn't commit," the ambassador told reporters after the hearing.
Reed, 30, is one of three Americans -- along with Brittney Griner and Paul Whelan -- waging high-profile fights to be freed from detention in Russia as the nation grows further isolated by the day from the rest of the world over its invasion of Ukraine.
Sullivan is concerned about Reed's health, after the former Marine was moved to a hospital with tuberculosis-like symptoms, the ambassador said. Reed participated in Tuesday's proceedings via videoconference.
"I am very concerned about his health. We appeal to the Russian government to treat him -- to treat his health problems -- promptly and tell him how he is being treated so that he can be reassured -- he and his family can be reassured -- that his health issues are being addressed," Sullivan said.
The situation in Ukraine, the ambassador said, "should have absolutely nothing" to do with the cases of jailed Americans in Russia.
"I hope that no one here in Russia, either in the Russian government or the Russian people, would think that there should be any different treatment for a person held in the criminal justice system here because of the tragic horrific events in Ukraine," he said.
Correction: An earlier version of this story gave the wrong age for Reed. He's 30.