Tehran, Iran (CNN) -- Concern is growing for the well-being of four journalists, three of them American citizens, apparently detained in Iran this week, according to The Washington Post.
Two freelance photojournalists, both American citizens, also have been detained, according to The Post. The paper said officials had not yet named the pair.
An Iranian official confirmed to CNN that Rezaian and Salehi are among three journalists being held by authorities. He did not say what they have been charged with.
The third journalist is a freelance photographer, the official said on condition of anonymity.
It's not clear why there is a discrepancy in the number of journalists held.
Gholam-Hossein Esmaili, the director general of the Tehran Province Justice Department, is quoted by Iran's official IRNA news agency as saying a "Washington Post journalist has been detained for some questions and after technical investigations, the judiciary will provide details on the issue."
He did not specify on what grounds the correspondent, whom he did not name, was being held.
Iranian security forces are vigilant about all kinds of enemy activities, Esmaili added, according to IRNA.
In a statement Thursday, Washington Post foreign editor Douglas Jehl said "we are deeply troubled by this news and are concerned for the welfare of Jason, Yeganeh and two others said to have been detained with them."
He described Rezaian as "an experienced, knowledgeable reporter who deserves protection and whose work merits respect."
The Committee to Protect Journalists said it was alarmed by The Post's report and urged Iran to release the four journalists.
"We call on Iranian authorities to immediately explain why Jason Rezaian, Yeganeh Salehi, and two other journalists have been detained, and we call for their immediate release," said Sherif Mansour, its Middle East and North Africa program coordinator.
"Iran has a dismal record with regard to its treatment of imprisoned journalists. We hold the Iranian government responsible for the safety of these four."
The Iranian mission to the United Nations in New York is following up on the case, spokesman Hamid Babaei said.
"We will inform the media as soon as we receive any news from Tehran," Babaei said.
According to The Washington Post, Rezaian, 38, holds American and Iranian citizenship. He has been the paper's Tehran correspondent since 2012.
His wife, an Iranian citizen who has applied for U.S. permanent residency, is a correspondent for the UAE-based newspaper the National, The Post said.
Members of Rezaian's family said they did not want to comment at this time, the paper added.
Thomas Erdbrink, Tehran bureau chief for The New York Times, said via Twitter, "I strongly condemn the arrest of my friend and colleague @jrezaian and his wife @YeganehSalehi, and two photographers, also friends."
Rallies are being held in Iran to mark Quds (Jerusalem) Day on what is the last Friday of the holy month of Ramadan.
Muslims worldwide mark this day each year to remember the plight of the Palestinians. Several foreign journalists are in Tehran to cover the event, which includes an anti-Israel demonstration after the Friday prayers.
According to CPJ research, there are 35 journalists in prison in Iran.
The organization also highlights past instances in which Iran has detained international journalists, including the case of U.S. freelancer Roxana Saberi.
CNN's Shirzad Bozorgmehr and Joe Sutton contributed to this report.