U.S. Secretary of State Hillary Clinton appealed to Iran for information on three Americans believed to have been arrested by Iranian authorities after entering Iran by mistake while on a hiking trip in neighbouring Iraq.

Clinton called on Tehran to help the United States determine their whereabouts and said the U.S. has also requested the help of Switzerland, which has represented U.S. interests in Iran since American diplomats were taken hostage at the embassy in Tehran 30 years ago.

"As of a few hours ago, we did not yet have official confirmation that the Iranian government or an instrument of the Iranian government were holding the three missing Americans," Clinton told reporters. "We asked our Swiss partners … to please pursue our inquiries to determine the status of the three missing Americans."

A Kurdish official in northern Iraq said the three contacted a colleague to say they had entered Iran by mistake on Friday and were surrounded by troops. Iran's state television later said the Americans were arrested after they did not heed warnings from Iranian border guards.

Cindy Hickey told The Associated Press that her son, Shane Bauer, was one of the three missing Americans. Hickey, who lives in Pine City, Minn., said she hopes the three return safely to the U.S.

"Our family is concerned about the safety and welfare" of the three, Hickey said. She refused to comment further.

Bauer is a freelance journalist and photographer based in the Middle East who has reported from Iraq, Syria, Sudan's Darfur region and Yemen, according to his website. He grew up in Minnesota and graduated from the University of California-Berkeley with a degree in peace and conflict studies, the site said.

One of the other missing Americans has been identified as Joshua Fattal. Fattal's mother, Laura Fattal of Elkins Park, Pa., issued a brief statement on Sunday expressing the family's concern.

The identity of the third American has not been confirmed, but other media reports have identified her as Sarah Shourd. Sandy Close, executive director of Pacific News Service where Bauer used to work, said one of Bauer's good friends told her Saturday that Shourd was taken into custody with Bauer, but she had no other details.

In a telephone interview, Hickey told the AP that her son was missing along with "Sarah and Josh," but she wouldn't elaborate further.

A fourth American, Shon Meckfessel, skipped the hike because he had a cold, his grandmother has said. The linguistics student called her Saturday from the U.S. Embassy in Baghdad, she said.

Swiss met with Iran on U.S. behalf

Swiss diplomats in Iran were asking officials from the Iranian Foreign Ministry for details but had not yet gotten official confirmation of the arrests, Clinton said. She asked that Iran determine the facts of the case and to "return them as quickly as possible."

"Obviously, we are concerned," Clinton said. "We want this matter brought to a resolution as soon as possible, and we call on the Iranian government to help us determine the whereabouts of the three missing Americans and return them as quickly as possible."

State Department spokesman P.J. Crowley told reporters later that the Swiss ambassador in Tehran met with Iranian officials Sunday. The Iranians told the Swiss that they could not confirm the detention of the Americans, Crowley said.

Crowley said the fourth American who did not accompany the missing hikers has provided good information to U.S. officials in Baghdad. He did not elaborate.

"We have three Americans who are missing. We want to know what happened to them," Crowley said. "Clearly, we want them back as quickly as possible."