Iranian security forces have arrested an Iranian-American businessman who had promoted improved ties between the two countries, adding to signs that hard-liners in Tehran are trying to block foreign investors from entering the Islamic Republic in the wake of the historic nuclear deal.

In the past few weeks, Iranian businessmen with links to foreign companies have been detained, interrogated and warned against wading into economic monopolies controlled by the Revolutionary Guard Corps, according to several businessmen interviewed inside and outside of Iran.

About two weeks ago, Siamak Namazi, the head of strategic planning at Crescent Petroleum Co., was arrested as he was visiting relatives in Tehran from his home base in Dubai, according to people briefed on the situation. He was arrested, they said, by the Guard’s intelligence arm, which reports to Supreme Leader Ayatollah Ali Khamenei, not the government.

Namazi’s arrest will further complicate the Obama administration’s ability to build better ties with Iran in the wake of the nuclear deal.

He is the fourth Iranian-American to be detained by the regime in recent years, three of whom, including Washington Post journalist Jason Rezaian, have been accused of espionage or subversion.

Secretary of State John Kerry is meeting with Iranian diplomats this week in Vienna to try and forge an agreement to end Syria’s civil war, in which Iran has been launching military operations in conjunction with Russia in support of President Bashar al-Assad.

Namazi’s friends say Iranian intelligence agents ransacked his family home in Tehran and confiscated his computer, and have since been launching cyberattacks on some of his email contacts.

Iran has also recently arrested a Lebanese information-technology specialist who lives in Washington and has permanent-resident status in the U.S.

Nizar Zakka, a well-known advocate of Internet freedom, had traveled to Tehran in mid-September at the invitation of the government to speak at a conference about entrepreneurship and met with several ministers, his family confirmed on Thursday. He was detained on his way to the airport in Iran, his wife said.

Tehran’s moves have fueled concern that hard-line political forces are cracking down on individuals and groups seeking a more open political and business environment since Iran signed the nuclear agreement with global powers in July.

Iran’s economy, in a shambles after years of sanctions, is in desperate need of investment.

President Hassan Rouhani and his more pragmatic-minded government have been locked in a power battle with hard-liners on the future direction of Iran. His administration has aggressively courted Iranian-American investors and has called on them to act as “a bridge for foreign companies.”

Hard-liners connected to the Revolutionary Guards and Khamenei have openly said that a nuclear deal should not translate to a red carpet for American companies, seeking to avoid accompanying social changes that could weaken their tight grip on the population.

Iran’s mission to the United Nations didn’t reply to questions about the recent crackdowns on executives or the arrests of Messrs. Namazi and Zakka. Neither of the men has had access to lawyers.

Namazi’s family declined to comment.

Click for more from The Wall Street Journal.