The US has called on Iran to immediately release American citizens detained on "fabricated" national security charges, on the same day Iran sentenced an American man to 10 years in prison.
The Center for Human Rights in Iran has named the man as Xiyue Wang, a 37-year-old Princeton University researcher of Chinese origin.
Separately, a US university professor who advised Xiyue has defended his former student as innocent of all charges against him.
Stephen Kotkin told the Associated Press news agency on Monday that Xiyue is a "remarkable, linguistically gifted graduate student" who studied governance in 19th- and early-20th-century Muslim regions.
Iran's judiciary announced on Sunday that the convicted American citizen had been accused of "infiltrating" Iran and passing confidential information to the US government and research institutions abroad.
But Kotkin said the documents Wang collected in Tehran were 100 years old.
"We call for the immediate release of all US citizens unjustly detained in Iran so they can return to their families," the Department of State 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.
Gholamhossein Mohseni Ejeie, Iran's deputy judiciary chief, said the man held dual nationality of the US and another country and had appealed to have his sentence overturned.
The Department of State 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."
'Espionage and collaboration'
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 US.
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.
An Iranian-American art dealer Karan Vafadari and his wife Afarin Niasari are also under detention.
The US and Iran have had no diplomatic relations since April 1980 in the wake of the 1979 Islamic revolution, and tensions have sharpened under US President Donald Trump after a brief warming under his predecessor, Barack Obama.
The two countries along with the other great powers signed an accord July 14, 2015, aimed at curbing Iran's nuclear programme.
The US has continued to honour the accord despite Trump's threats as a candidate last year to "rip it up".
But in mid-June, the US Senate approved for new sanctions against Iran, denouncing the Islamic republic's "continued support of terrorism".
The bill must be passed by the House of Representatives.
The new restrictions impose mandatory sanctions on people involved with Iran's ballistic missile programme and those that transact with them.
Iran has vowed to respond with "reciprocal and adequate measures", while accusing the US of violating the nuclear deal signed between Iran and world powers in 2015.
Source: Al Jazeera and news agencies