An Iranian security official confirmed on Tuesday the arrest of three Americans for "illegal entry" into Iran near the border with Iraq and said they are being interrogated.

"The Azmar mountains north of the Iraqi Kurdistan city of Sulaimaniyah. An Iranian security official has confirmed the arrest of three Americans for "illegal entry" into Iran near the border with Iraq and said they are being interrogated."

The detentions come as Washington is seeking to increase pressure on Tehran over its nuclear drive and amid deep political turmoil in Iran following President Mahmoud Ahmadinejad's disputed re-election in June.

"These three people whose identity is not known yet were arrested on the Malakh-Khor border near the town of Marivan," Iraj Hassanzadeh, deputy governor for security in Iran's Kordestan province, told the Fars news agency.

"Anyone who seeks to illegally enter the country from the Kordestan border will be arrested," he said, referring to an area near the frontier with the Kurdish area of northern Iraq.

U.S. media have identified the trio who went missing on Saturday while backpacking in the rugged mountains between Iraq and Iran as Shane Bauer, Sara Shourd and Joshua Fattal -- all in their mid-20s.

Hassanzadeh told Iran's Arabic-language Al-Alam television that the three were arrested "four days ago and are being interrogated."

He told Fars that the detainees had Syrian and Iraqi visas and that "they have made no confessions yet."

Iranian state television said various foreign media outlets had presented the three Americans as tourists, hikers, journalists and even "CIA agents."

U.S. Secretary of State Hillary Clinton called Monday on Iran to ensure their safe return.

Switzerland, which in the absence of U.S.-Iranian diplomatic relations represents American interests in Iran, had been asked to help track them down, she added.

American media said Bauer, a photo-journalist, Shourd, an English teacher and Fattal, an adventurer with a passion for travel, had all travelled to the Middle East late last year, although it was unclear if they had gone together.

On this trip, a fourth American travelling with them had decided to stay behind in a hotel in the Kurdish region's second largest city of Sulaimaniyah.

"Pedestrians wait by the closed gates on the Iraqi side of the Iraq-Iran border crossing of Haj Omran in 2007. An Iranian security official has confirmed the arrest of three Americans for "illegal entry" into Iran near the border with Iraq and said they are being interrogated."

Bauer is "a gifted writer and photographer who was fluent in Arabic and very interested in Arabic culture," said Annette Fuentes, managing editor of New American Media (NAM), to which Bauer has been a regular contributor since last year.

The last time NAM heard from Bauer was at the end of July, when he sent the media outlet an email saying he was heading to Kurdistan to cover the elections in the autonomous region.

"We were expecting to get his story and instead we learned that this had happened," Fuentes said.

"We are anxious that he will be safe and that there will be a resolution and any misunderstanding about why he was there and what he was doing will be cleared up and he will be allowed to go about his way," she said.

Shourd, who is Bauer's girlfriend, has also had a story published by NAM.

Iran has in past jailed foreign nationals for illegal entry into its territory.

Iranian-American reporter Roxana Saberi, who had been living and working in Iran, was sentenced earlier this year to eight years in prison for spying for the United States but was freed after serving 100 days in jail.

In November 2005 Iran arrested a German national and a Frenchman after straying into its territorial waters in the Gulf during a fishing expedition.

The two were sentenced to 18 months in jail and released in 2007 after being granted clemency by supreme leader Ayatollah Ali Khamanei.

Mystery still surrounds the fate of former FBI agent Robert Levinson, who went missing on the Iranian island of Kish in the Gulf in March 2007.