The White House says the United States is "deeply concerned" about three American hikers held in Iran since July.
Their families say Shane Bauer, 27, Josh Fattal, 27, and Sarah Shourd, 31, are in poor health and considering a hunger strike.
A spokesperson for President Barack Obama says the White House is "deeply alarmed" to learn of their deteriorating state of physical and emotional health, and urged the government of Iran "to release them immediately."
Iran said earlier this month it had proof the three - detained on espionage charges - had links to intelligence services.
Swiss diplomats visited the detainees on Thursday.
The trio was detained on July 31 after straying across the border during a hiking trip in the Kurdistan region of Iraq. They are being held in Evin prison in Tehran.
In December, Foreign Minister Manouchehr Mottaki said Iran will put them on trial after they "illegally" entered the country.
