An unidentified man leaves a Dassault Falcon jet of Swiss air force at the airport in Geneva, Switzerland, Sunday, Jan 17, 2016 in front of a US government plane. Another US aircraft waited nearby to bring back to the US the men who were left from imprisonment in Iran the day before. (AP Photo/Laurent Cipriani)

U.S. citizens traveling to Iran, particularly Iranian-Americans, risk arrest and detention in the Islamic republic, the U.S. State Department said in a travel warning on Friday.

The notice, which largely echoes an August warning, was issued days after five Americans, including four dual Iranian nationals, were released by Iran in a prisoner swap coinciding with the lifting of economic sanctions on Iran as the implementation of a deal to curb its nuclear program began.

The warning aims to “reiterate and highlight the risk of arrest and detention of U.S. citizens, particularly dual national Iranian-Americans, in Iran,” the State Department said.

“Various elements in Iran remain hostile to the United States,” it said, adding that Iran has “continued to harass, arrest, and detain U.S. citizens, in particular dual nationals” since the nuclear deal was signed in July.

In his first appearance since returning to the United States, one of the prisoners freed in the swap, Washington Post reporter Jason Rezaian, thanked U.S. Secretary of State John Kerry on Thursday for negotiating his release after 18 months in an Iranian jail.

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