The Committee to Protect Journalists (CPJ) has called on the Iranian government to immediately release four detained journalists, three of whom it said had US-Iranian nationality.
Two of the detainees are Jason Rezaian, the Tehran correspondent for the Washington Post, and his wife Yeganeh Salehi, a correspondent for the United Arab Emirates-based newspaper the National, the CPJ said in a statement.
"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 CPJ Middle East and North Africa Program Coordinator Sherif Mansour.
"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.
"Names of the other two detained journalists have not been revealed but the Post said they were "freelance photojournalists".
Rezaian, 38, has dual US-Iranian nationality and has worked for the Post in Tehran since 2012. The Post reported that Salehi "has applied for US permanent residency".
US State Department deputy spokeswoman Marie Harf said Washington was aware of the reports of the detention of three US citizens in Iran.
"We are not able to comment further at this time due to privacy considerations. Our highest priority is the safety and welfare of US citizens abroad," she said.
Iran does not recognise dual citizenship.
"We have received credible reports that Rezaian and his wife Salehi were detained on Tuesday evening in Tehran," said Douglas Jehl, foreign editor of the Washington Post.
Besides the four, there are 35 journalists in prison in Iran, according to CPJ.
'Persecuted for religious activities'
UN special rapporteur for human rights in Iran, Ahmed Shaheed, said in March that Iran was holding almost 900 political prisoners, "including people persecuted for religious activities, lawyers and journalists".
In his latest report to the UN Human Rights Council, Shaheed said there were 379 political activists, 292 religious practitioners, 92 human rights defenders, 71 civic activists, 37 journalists and bloggers and 24 students held as what he defined as political prisoners.
Iran denies Western allegations that it is trying to stifle dissent by arresting elites, including senior moderate politicians, activist students, lawyers and journalists.
The government says it welcomes constructive criticism and upholds the principle of free speech.
President Hassan Rouhani's landslide election win last year raised hopes among activists that he would create a freer society, but analysts and rights activists say so far Rouhani has failed to fulfill his campaign promises on political freedoms.
Iran detained four Iranian-American academics in 2007 for four months for allegedly spying.
An Iranian-Canadian journalist, Maziar Bahari, and an Iranian-American journalist, Roxana Saberi, were detained in Iran for four months in 2009 also on spying charges.