The White House said it was making efforts to free two detained Americans from Russia as it voiced indignation at Evan Gershkovich being put on trial.
As Russia said it was sending "signals" to the United States about a possible prisoner swap, National Security Council spokesman John Kirby said that Moscow was using both Mr Gershkovich and fellow detained American Paul Whelan "as a bargaining chip."
"We want both Evan and Paul to know that we all care deeply about their well-being and that this administration will continue to do everything it can to bring them home," Mr Kirby told reporters.
He did not detail diplomacy with Russia on the two prisoners but said, "It's alive, and we're keeping at it."
Mr Kirby said that Russia allowed the US embassy only "brief access" to Mr Gershkovich during his "sham trial" in the Urals city of Yekaterinburg.
"I'll state it again - Evan has never been employed by the United States government. Evan is not a spy. Journalism is not a crime, and Evan should never have been detained in the first place," Mr Kirby said.
Mr Gershkovich went on trial behind closed doors in the Russian city of Yekaterinburg, where he faces charges of espionage and a likely sentence of up to 20 years in prison.
Prosecutors say the Wall Street Journal reporter gathered secret information on the orders of the US Central Intelligence Agency about a company that manufactures tanks for Russia's war in Ukraine.
Mr Gershkovich, his newspaper and the US government all reject the allegations and say that he was just doing his job as a reporter accredited by the Foreign Ministry to work in Russia.
"This bogus accusation of espionage will inevitably lead to a bogus conviction for an innocent man," Wall Street Journal editor-in-chief Emma Tucker said in a letter to readers.
The US embassy said in a statement: "His case is not about evidence, procedural norms, or the rule of law. It is about the Kremlin using American citizens to achieve its political objectives."
The Kremlin declined to comment on the opening of the trial. Spokesman Dmitry Peskov told reporters: "We know that this topic is very, very resonant in the United States, but it is not so resonant within our country."
After several hours of closed proceedings, the court said the next session would take place on 13 August - an indication the case will drag on for months.
Journalists were briefly allowed to film the 32-year-old before the start of the trial, from which the media are barred.
The reporter was shaven-headed, in contrast to previous court appearances, and wore an open-necked shirt. He smiled and nodded at colleagues he recognised.
Prosecutor Mikael Ozdoyev spoke briefly to journalists to summarise the charges. "Gershkovich carried out the illegal actions while observing painstaking conspiratorial measures," he said.
US reporter's trial held in secret
Closed trials are standard in Russia for alleged treason or espionage involving classified material.
The politicians sign non-disclosure agreements, preventing the emergence of any details on Mr Gershkovich's alleged actions and how he will defend himself.
The Kremlin said the case and the trial arrangements are a matter for the court but has stated - without publishing evidence - that Mr Gershkovich was caught "red-handed".
Against the background of the Ukraine war, he and other Americans detained in Russia have been caught up in the gravest crisis between Moscow and Washington for more than 60 years.
President Vladimir Putin has said Russia is open to a prisoner exchange involving Mr Gershkovich and that contacts with the United States have taken place but must remain secret.
The US Embassy said Russian authorities had failed to provide evidence supporting the charges against Mr Gershkovich or to explain why his work as a journalist constituted a crime.
Gershkovich alleged to have researched tank maker
The trial is taking place in Yekaterinburg, 1,400 km east of Moscow, where officers of the FSB security service arrested Mr Gershkovich on 29 March 2023, while he was eating in a steakhouse.
He has spent nearly 16 months in Moscow's Lefortovo prison.
The Wall Street Journal has declined to comment on the purpose of his reporting trip to the Urals region or on the specific allegation that he was seeking information on Uralvagonzavod, a supplier of tanks for Russia's war in Ukraine.
"He was there as an accredited journalist, doing his job," Wall Street Journal publisher Almar Latour told Reuters in a phone interview before the trial.
Many Western news organisations pulled staff out of Russia after it sent its army into Ukraine in February 2022 and Moscow passed laws that set long prison terms for "discrediting" the armed forces or spreading "fake news" about them.
Mr Gershkovich was one of a small number of Western reporters, also including journalists from Reuters, who continued to report from inside Russia.
Russia said in the first weeks after his arrest that any exchange could only take place after a trial.
Kremlin spokesman Mr Peskov reiterated that contacts with the US on a possible deal required "silence" and Moscow would not speak publicly about them.
