An Iranian-American businessman who advocated better ties between Iran and the U.S. reportedly has been arrested and imprisoned in Tehran, becoming the fourth U.S. citizen known to be held there at a time when hard-liners are pushing back against the country's nuclear deal with world powers.

The reasons for the reported detention of Siamak Namazi were not immediately known, though hard-liners in Iran's judiciary, military and intelligence services have warned against easing hostilities with the U.S. On Friday, Iran and the U.S. sat down together in new multilateral talks with other countries, trying to end Syria's long civil war.

In addition to Namazi's detention, an Internet freedom group said a Washington-based Lebanese citizen recently disappeared while on a trip to Tehran.

Namazi, who is believed to be in his 40s, was arrested earlier this month while visiting Tehran, according to reports in The Wall Street Journal, The New York Times and The Washington Post, all of which relied on anonymous sources. Attempts by The Associated Press to reach family members of Namazi, who has a home in Washington, weren't immediately successful Friday.

Iranian officials could not be reached for comment Friday, part of the Iranian weekend, and state media did not report on Namazi's reported detention.

The U.S. State Department declined to confirm Namazi's arrest.

"We're aware of recent reports of the possible arrest in Iran of a U.S. citizen. We're looking into these reports and don't have anything further to provide at this time," Michael Tran, a State Department spokesman, said late Thursday.

Namazi is a son of Bagher Namazi, a former UNICEF representative who once served as governor of Iran's oil-rich Khuzestan province under the U.S.-backed Shah Reza Pahlavi. He later fled as a child with his family after the 1979 Islamic Revolution. But the family appears to have kept business ties in Iran, and Namazi travelled back several times.

His last known employer was Crescent Petroleum, a firm based in Sharjah in the United Arab Emirates. A representative of the company said Namazi traveled to Tehran in mid-July on a private visit and "at the time of his reported detention was no longer an employee of the company."

The representative declined to be identified publicly, citing the political sensitivities surrounding the issue.

A consulting company in Iran with which Namazi previously had ties, Atieh Bahar, denied to AP that he had any ongoing role there. An official, who refused to be identified, said Namazi had not worked with the firm "for eight or 10 years."

In online articles he wrote for Iranian.com, Namazi says he graduated from Tufts University in 1993, then returned to Iran for a time. "This land of my dreams was all I thought about. My passion for return was so great, it hurt," he wrote in 1997. "In Iran, you are either experiencing heaven or hell. There are very few in-betweens."

In recent years, Namazi wrote several pieces calling for improved ties between Iran and the United States and urging Iranian-Americans to act as a bridge between the rival governments. In 2013, Namazi wrote about medical supply shortages caused by international sanctions on Iran in an opinion piece published by The International Herald Tribune. He urged the West to "relax and rationalize" the sanctions to allow medical goods into Iran, saying otherwise "more Iranian men, women and children will suffer needlessly."

Still, Namazi raised suspicions among hard-liners in Iran. In May, a hard-line Iranian website called Fardanews specifically pointed to him in a highly critical article, accusing him of being part of efforts to allow the West to infiltrate Iran.

"Siamak has always played the role of bridge between the U.S. government with some notorious local figures in the country," the website said. "His allies and family have been ground-preparing for soft infiltration of the U.S. in Iran."

Namazi's reported arrest comes as hard-liners in Iran remain opposed to a detente with the United States in the wake of Iran's nuclear deal with world powers. That agreement reached earlier this year promises Iran relief from crippling economic sanctions in exchange for curbs on its nuclear program.

Iranian hard-liners are opposed to moderate President Hassan Rouhani's strategy of attempting to improve ties with the West. Internal domestic struggles over the direction of Iran appear to be intensifying ahead of February's parliamentary elections.

There also may be another aim: In August, Iranian state media began quoting officials discussing the possibility of swapping Americans detained in Iran for 19 Iranians held in the U.S. However, it's unclear whether that's been seriously discussed between Iranian and American officials.

In the past few weeks, Iranian businessmen with links to foreign companies have been detained, interrogated and warned against becoming involved in economic monopolies controlled by Iran's powerful Revolutionary Guard, The Wall Street Journal reported, citing businessmen inside and outside of Iran.

Lebanese citizen Nizar Zakka disappeared Sept. 18 while visiting Tehran for a state-sponsored conference, according to a statement issued by the Washington-based group IJMA3-USA, which advocates for Internet freedom across the Middle East. Zakka was last seen leaving his hotel in a taxi for the airport to fly to Beirut, but never boarded his flight, according to the statement signed by lawyer Antoine Abou Dib.

Calls to IJMA3-USA and a phone number for Abou Dib rang unanswered early Frida. It is not known why Iranian authorities would want to detain Zakka remains unclear, though The Wall Street Journal said he held permanent-resident status in the U.S.

Iran announced earlier this month that The Washington Post's Jason Rezaian was convicted by Iran's Revolutionary Court on charges including espionage, though there have been no details on the verdict or sentence. He was detained in July 2014 and has now been in detention longer than the 52 American diplomats and citizens who were held hostage in Iran for 444 days after the country's Islamic Revolution.

Other Americans held in Iran include former U.S. Marine Amir Hekmati, who holds dual Iranian and American citizenship and was arrested in August 2011. Saeed Abedini, a pastor from Boise was convicted in 2013 of threatening Iran's national security by participating in home churches. The U.S. also says it has asked for the Iranian government's assistance in finding former FBI agent Robert Levinson, who went missing in 2007 while working for the CIA on an unapproved intelligence mission.

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Associated Press writer Nasser Karimi in Tehran, Iran, contributed to this report.

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