WASHINGTON — The State Department says Americans should think twice about any nonessential travel to Iran.

A new travel warning highlights the danger to Americans of arrest and detention in Iran. The alert comes 12 days after the U.S.-Iranian prisoner swap that freed five American citizens.

Four were dual Iranian-American citizens. The department says such people are especially at risk.

The alert says "various elements" in Iran remain hostile to the U.S. despite last summer's nuclear deal. It says Iranian authorities continue to "harass, arrest and detain U.S. citizens."

Because the two countries don't have diplomatic relations, the U.S. is limited in the help it can provide Americans there. The U.S. generally operates through Swiss intermediaries. But Iran often denies the Swiss consular access to dual nationals.