Iran's confirmation Sunday that it has detained a fourth Iranian-American seems certain to further rile relations between the two countries.
The United States has sharply criticized the detentions but Iran insists America has no right to interfere.
Mohammad Ali Hosseini, the spokesman for Iran's foreign ministry, confirmed at his weekly news briefing that Iranian-American Ali Shakeri had been detained.
It was the first official confirmation, although the semiofficial ISNA news agency on Friday reported that Shakeri, of Lake Forest, California, was being held and investigated by the security department of the Tehran prosecutor's office.
Shakeri, founding board member of the University of California, Irvine, Center for Citizen Peacebuilding, is the fourth dual citizen detained in Iran in recent months.
Iranian officials previously confirmed the detentions of three other Iranian-Americans: scholar Haleh Esfandiari, who is the director of the Middle East program at the Woodrow Wilson International Center for Scholars; Kian Tajbakhsh, an urban planning consultant with George Soros' Open Society Institute; and journalist Parnaz Azima, who works for U.S. funded-Radio Farda.
All three have been accused of endangering Iran's national security and of espionage, according to a judiciary spokesman. It is not known if Shakeri has been accused of specific wrongdoing.
They were in Iran visiting family members or engaged in professional work, according to the U.S. State Department and their relatives and employers.
U.S. President George W. Bush has demanded that Iran "immediately and unconditionally" released them, and has denied that they were spying for the United States. Family, colleagues and employers also denied the allegations.
Bush's remarks have drawn sharp criticism from Iranian officials, with Tehran accusing Bush earlier this week of interfering in Iran's internal affairs.
Iran has also escalated accusations against the United States, saying last month it uncovered spy rings organized by the U.S. and its Western allies.
Meanwhile, five Iranian officials detained in the northern Iraqi city of Irbil by U.S. troops in January, remain in U.S. custody. The U.S. military has said they are suspected of links to a network supplying arms to Iraqi insurgents - an accusation that Iran has denied.
In an interview with The Associated Press this week, U.S. Secretary of State Condoleezza Rice said that as unwarranted as the current detentions are, they would not stop the United States from trying to engage Iran on other matters, including its disputed nuclear program and alleged support of insurgents in Iraq.
"We take seriously the holding of any American anywhere in the world where they are being wrongly held and where they are being accused of things that clearly are untrue," Rice said. "It just shows again what kind of regime this is."
But the detentions appear to have further cast in doubt what was expected to be a second round of direct American-Iranian talks on Iraq this month. The talks have been billed as a possible window to better relations.
The U.S. and Iranian ambassadors in Iraq met last month in Baghdad and although they limited the agenda to Iraq's instability, the talks were groundbreaking, as the first formal diplomatic meeting in nearly three decades.
After the meeting, Iranian Ambassador Hassan Kazemi Qomi said the two sides would meet again in less than a month.
In the interview with The AP, Rice said the United States has not yet determined "when and if it makes sense" to continue the dialogue with Iran.
On Sunday, Tehran also cast doubt on whether the talks would take place.
Hosseini, the foreign ministry spokesman, said that Iran had not agreed to a second round and that the Iranians were studying the results of the May 28 talks and would decide later whether to continue them.
Foreign Minister Manouchehr Mottaki later signaled Iran would be willing to continue the talks. "Iraq should not be a scene for settling scores with any country. All should help in removing current problems of (Iraq)," Mottaki said in veiled criticism of the U.S.
The U.S. has also expressed concern about former FBI agent Robert Levinson, who the United States says has been missing since March after traveling to an Iranian resort island on private business.
Hosseini reiterated Sunday that Iran has no information about Levinson.
Hosseini also accused the United States of using scientific and research cooperation as a guise to work against Iran. It was not clear what he referred to, but many academics have criticized Iran for arresting scholars.
The United States broke off ties to Tehran after the storming of the U.S. Embassy there in 1979 and the seizure of U.S diplomats as hostages. Iran held the Americans for 444 days, and the episode sealed Iran as the principal U.S. adversary in the Middle East.
But despite the tensions, neither side is looking for a military confrontation. During a visit Sunday to Kuwait, Iranian Parliament Speaker Gholam Ali Haddad Adel said it was "unlikely" Washington would attack his country, but if it happened, "we would have to defend ourselves."
Adel, speaking through an interpreter, said he also doubted regional countries would allow the U.S. to launch military attacks on Iran from their lands, because they wouldn't want to "tie their fate to mistakes America makes," a likely reference to Iraq.