(CNN) — Wall Street Journal Editor-in-Chief Emma Tucker told staff in a memo Thursday that she was "very concerned" for the safety of Russia-based reporter Evan Gershkovich, who was arrested Wednesday by security forces in the city of Yekaterinburg.

"I am very sorry to let you know you that one of our reporters Evan Gershkovich was yesterday detained outside Moscow according to a statement put out by the Russian security services," Tucker wrote in a memo to newspaper staff at 5:11 am ET and later obtained by CNN.

"We are very concerned for the safety of Evan and will keep you informed of the situation," Tucker continued.

Gershkovich was arrested in Russia on suspicion of espionage, the first time a US journalist has been detained on accusations by Moscow of spying since the Cold War.

In a statement, the Wall Street Journal said it "vehemently denies the allegations from the FSB and seeks the immediate release of our trusted and dedicated reporter, Evan Gershkovich."

"We stand in solidarity with Evan and his family," the newspaper said.

The National Press club on Thursday called on Russia to immediately release Gershkovich, calling his detention "unjust," and is also urging action from the State Department.

"Evan Gershkovich is a journalist. He should be released immediately and unharmed and allowed to return to his important work," said Eileen O'Reilly, president of the National Press Club, and Gil Klein, journalism president of the National Press Club, in a statement.

The group, described as a global organization for journalists, said that in addition to considering this an unjust detention, it's calling on the State Department to "designate his detention in that manner at once."

CNN has contacted the US State Department for comment.

CNN's Kristina Sgueglia contributed to this report

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