U.S. reporter Evan Gershkovich on Thursday lost his latest appeal of his pretrial detention in Russia on charges of espionage.

The Wall Street Journal reporter denies the spying charge, which could lead to a prison sentence of up to 20 years if he is convicted.

U.S. Ambassador Lynne Tracy, who was not allowed in the Moscow courtroom for the hearing, told reporters outside that she was "extremely disappointed" by the decision.

She praised Gershkovich, 31, for his "remarkable strength and resiliency" and reiterated the U.S. stance that the charges against him were baseless.

"He is an innocent journalist who was carrying out journalistic activities and has been wrongfully detained. Such hostage diplomacy is unacceptable," Tracy said.

The Kremlin has said Gershkovich was caught "red-handed" on a trip to the Urals city of Yekaterinburg, where the FSB security service said he was trying to obtain military secrets. It has provided no evidence to support that assertion, which is vehemently denied by The Wall Street Journal.

The reporter's parents, Mikhail Gershkovich and Ella Milman, who left the Soviet Union for the United States in 1979, were in court to support their son. They left afterward without speaking to journalists.

Gershkovich, wearing blue jeans and a black T-shirt, stood in a glass box and smiled at reporters who were briefly allowed to film him before the start of the proceedings. He was appealing his detention in Moscow's Lefortovo prison ahead of his trial, for which no date has been set.

Emma Tucker, the Journal's editor in chief, told the BBC before the hearing that she had low expectations from the appeal but it was important to go through the legal process.

In April, a court denied an earlier request from Gershkovich's lawyers that he be given house arrest, agree to restrictions on his movements, or be granted bail.

He is being held at a time when relations between Washington and Moscow are at the lowest point since the Cold War after Russia launched what it calls its "special military operation" in Ukraine.

Ambassador Tracy said Washington demanded the immediate release of Gershkovich and Paul Whelan, a former U.S. marine who was arrested in Russia in 2018 and jailed for 16 years in June 2020 on spying charges.

He too is designated by Washington as wrongfully detained, a term that means the United States considers the verdicts to be bogus and politically motivated