The United States has called on Iran to immediately release American citizens detained on "fabricated" national security charges, on the same day Tehran sentenced an American man to 10 years in prison.
"We call for the immediate release of all US citizens unjustly detained in Iran so they can return to their families," the State Department said on Sunday.
The statement made no specific reference to any individual, but it came after an Iranian spokesman announced the sentencing of a US citizen for alleged "infiltration" of the country.
Out of sight: Imprisoned in Iran
Authorities did not identify the American man, but Iran's deputy judiciary chief Gholamhossein Mohseni Ejeie said the man holds a dual nationality of the US and another country.
He also said the American individual has appealed his sentence.
The Center for Human Rights in Iran named the man as Xiyue Wang, a 37-year-old Princeton University researcher of Chinese origin.
The State Department said the detention of US citizens and other foreigners are based on "fabricated national security-related charges".
"The safety and security of US citizens remain a top priority. All US citizens, especially dual nationals considering travel to Iran, should carefully read our latest travel warning."
In January, Tehran's chief prosecutor said as many as 70 "spies" were serving sentences in the city's prisons -- the identities of only a handful of which have been made public.
Most are thought to be Iranians who hold another passport from Europe or the United States.
'Espionage and collaboration'
In October, US-Iranian business consultant Siamak Namazi and his 80-year-old father Baquer, a former UNICEF official, were given 10 years in prison for "espionage and collaboration with the American government".
The US has repeatedly called for their release and also for Tehran's cooperation on the case of Robert Levinson, a former FBI agent who disappeared in Iran in 2007.