The State Department issued an updated travel warning on Monday for Americans, especially Iranian-Americans, on the risks of travel to Iran, even after the U.S. signed a nuclear deal and made a $400 million cash payment to the country that was contingent on the release of four American prisoners.

The travel warning replaces what was in effect since March 14, which reiterates the risk of arrest and detention of U.S. citizens, especially those with dual Iranian-American citizenship. Iran does not recognize dual nationalities.

The warning says that Iranian authorities continue to "detain and imprison U.S. citizens, particularly Iranian-Americans, including students, journalists, business travelers, and academics, on charges including espionage and posing a threat to national security."

"Iranian authorities have also prevented the departure, in some cases for months, of a number of Iranian-American citizens who traveled to Iran for personal or professional reasons," the warning continues. "U.S. citizens traveling to Iran should very carefully weigh the risks of travel and consider postponing their travel.

Since the U.S. government does not have any diplomatic or consular relations with Iran, the U.S. has "extremely limited" abilities to help any Americans in the country in case of an emergency. Currently the Swiss government, acting through its embassy in Tehran, represents U.S. interests in Iran.

Last month, Iran's judiciary confirmed the detention of an Iranian-American who was visiting family in Iran, the country's semi-official ISNA news agency reported.

The report did not name the Iranian-American involved or say when he was arrested. It quoted the spokesman for the judiciary, Gholam Hossein Mohseni Ejehi, as saying that the man was arrested in the city of Gorgan on unknown charges and then referred to the Iranian capital for investigation.  The State Department said it was looking into reports that Iranian-American Robin Shahini was been detained in Iran.

His girlfriend told the Associated Press she was worried Shahini was arrested over online comments criticizing Iran's human rights record, adding that he was arrested in Gorgan, where he was visiting his family.

She said that Shahini's sister told her Iranian authorities took him into custody on July 11 and that he has not been heard from since. Shahini, 46, left Iran in 1998 and lived in San Diego. He graduated in May from San Diego State University.

In recent months, there have been three dual nationals and a Lebanese man who have been detained in Iran. The four, who have ties to Britain, Canada and the United States, all are believed to have been detained by Iran's paramilitary Revolutionary Guard, a hard-line force charged with ensuring the country's Islamic government remains in power. The charges they face remain unclear.

The latest detentions come as the Obama administration admitted last week that a $400 million cash payment to Iran in January was contingent on the release of American prisoners being held in the country – while still denying that the payment was a ransom.

State Department spokesman John Kirby said that the negotiations to return the money – originally from a 1979 failed military equipment deal made between Iran and the U.S. – were conducted separately from negotiations to free the four prisoners.

The four detainees who were released on January 17 were Washington Post reporter Jason Rezaian; former U.S. Marine Amir Hekmati; Christian pastor Saeed Abedini and Nosratollah Khosravi-Roodsari, whose case had not been publicized before the release.

However, Kirby said that the U.S. withheld the cash delivery until Iran made good on its promise to release the prisoners.

The new details, first reported by The Wall Street Journal, added to criticisms from Republicans that it was a ransom paid by the Obama administration.

Abedini has claimed that he and the other hostages were kept waiting at an Iranian airport for more than 20 hours before their departure. Abedini said he was told by a senior Iranian intelligence official that their departure was contingent on the movement of a second plane.

State Department officials denied Abedini's claims to the Journal, saying the delay was due to issues locating Rezaian's wife and mother, who accompanied him on the flight.

According to the Journal, GOP leaders say they plan to hold hearings on the payment next month, when Congress returns from its summer recess. 

The Associated Press contributed to this report.