American journalist Evan Gershkovich, who was arrested and charged with espionage in Russia last week, has appealed against his pre-trial detention through his lawyers, as Washington said it was "pushing hard" to secure his release.
A Moscow court last Thursday remanded the Wall Street Journal reporter in pre-trial detention until May 29 on charges that carry a prison term of up to 20 years.
Gershkovich's defence team has appealed the ruling that he be held at Moscow's Lefortovo prison pending trial.
The Wall Street Journal has denied the charges against Gershkovich, labelled them a "vicious affront to a free press" and called for his immediate release.
The Kremlin said last week, without providing evidence, that Gershkovich was "caught red-handed".
National Security Council spokesperson John Kirby told reporters on Monday the U.S. "will do everything we can to get Evan home" and said Washington had been "pushing hard since the moment we found out the reporter was detained."
Kirby did not comment on whether or when the United States would make the determination that Gershkovich was wrongfully detained.
"The State Department has a process here for classifying detentions and they are still working their way through that... It's case by case, when they look at detention of Americans overseas, and whether or not to classify them as wrongful. They're working this and I know they're working at it very hard. I just couldn't give you a timeline of what that's going to look like," he said.
The case is the most serious move against an American reporter in Russia since the end of the Cold War.
A state-appointed official said on Monday he had visited Gershkovich at Lefortovo prison, the main pre-trial jail for high-profile suspects and those charged with espionage or treason.
He has not been able to communicate with the outside world since he was arrested in the Urals city of Yekaterinburg last week. His lawyer was not able to see case materials during last week's hearing.
Wall Street Journal Editor-in-Chief Emma Tucker told The Times on Monday she was "hopeful" Gershkovich would be able to speak to a lawyer "sometime next week".
Russian U.N. Ambassador Vassily Nebenzia told reporters on Monday that the investigation was ongoing and he did not have any information about what evidence Russian authorities have about Gershkovich's actions.
When asked about consular access, Nebenzia said: "I think that according to consular and diplomatic traditions, it will be provided but at what stage and when I also have no idea - that is for the authorities in Moscow to decide."
(Reporting by Reuters; Editing by Kevin Liffey, Jonathan Oatis, Nick Macfie and Lisa Shumaker)
