Gershkovich has firmly rejected the charges, which carry a maximum sentence of 20 years in prison.
Any trial could be months away.
Gershkovich, who has also worked for AFP, is the first foreign journalist arrested on spying allegations since the collapse of the Soviet Union.
Since his arrest on March 29, Gershkovich has appeared in court only once before – at a closed custody hearing on March 30.
He was remanded in custody until May 29 and is being held at the Lefortovo prison in Moscow, where many high-profile prisoners accused of treason and espionage have been held.
“He is in good health and remains strong,” US ambassador Tracy was quoted by the US embassy as saying after visiting him on Monday.
‘Not losing hope’
In his first contact with the outside world, Gershkovich wrote a handwritten letter to his parents in Russian. “I am not losing hope,” it read.
His mother Ella Milman said he “felt it was his duty to report” from Russia.
“He loves Russian people,” she said in a video interview with the Wall Street Journal.
US President Joe Biden has called his imprisonment “totally illegal”.
More than three dozen news organisations have also signed a letter to the Russian ambassador in the United States, denouncing “unfounded espionage charges”.
“Gershkovich’s unwarranted and unjust arrest is a significant escalation in your government’s anti-press actions,” the letter released by the Committee to Protect Journalists said.
“Gershkovich is a journalist, not a spy, and should be released immediately and without conditions,” it added.
The arrest has raised speculation that Russia may want a prisoner swap like the one last year in which Moscow released US basketball star Brittney Griner, who had been arrested over traces of cannabis found in her possession.
She was exchanged for Viktor Bout, a Russian arms dealer imprisoned in the United States.
