Germany and Iran are locked in a tense diplomatic conflict over two Germans arrested in Iran while interviewing the son of a woman whose sentence to death by stoning drew widespread international condemnation.

The two men -- according to media reports journalists working for a large German newspaper -- were arrested Sunday while interviewing Sajjad Ghaderzadeh, the son of Sakineh Mohammadi Ashtiani, whose stoning sentence for adultery sparked outrage this summer. The Iranian government has accused the men of having done research without official permission and of connections to a foreign "counter-revolutionary group."

Authorities apparently also detained Ghaderzadeh, 22, and Ashtiani's lawyer Houtan Kian, who sat in on the interview in the northwestern city of Tabriz.

"We don't know where the four are, we have no contact to them, so we are very worried," said Mina Ahadi, the fifth person present during the interview. An exiled Iranian anti-stoning activist living in Germany, she was tuned in via telephone to act as a translator.

"Some 15 minutes into the interview, I heard strange noises and one of the German journalists said: 'What's going on here?' Then there was a whisper: 'Mrs. Ahadi, I have to hang up,'" she told United Press International in a telephone interview Thursday.

German Chancellor Angela Merkel has appealed for release of the two Germans. Guido Westerwelle, Germany's foreign minister said he had contacted his Iranian counterpart Manouchehr Mottaki to urge him to personally take up the case. "We are using all diplomatic levels," to free the Germans as quickly as possible, Westerwelle added.

Yet Iran isn't wavering.

Reminiscent of the affair surrounding the arrest of three Americans hikers by Iranian border guards last summer, the case of the two Germans threatens to turn into a major diplomatic conflict between Tehran and Berlin.

Relations have long been troubled by the conflict surrounding Iran's nuclear program, and Germany has recently come under pressure to reduce economic cooperation with Iran.

German companies have already begun to pull out of the country, yet significant economic and finance ties remain despite international sanctions against large parts of the Iranian industry.

Ashtiani's case is highly controversial -- when the stoning sentence surfaced, several high-ranking politicians, including U.S. Secretary of State Hillary Clinton, condemned it.

Iran has since shelved the stoning, but not the death sentence, saying Ashtiani was also convicted of having helped kill her husband.

Germany so far abstained from all-too harsh condemnations over the recent arrests and has instead dispatched a team from the German Embassy in Tehran to try to locate the detained.

Ahadi, who says her husband was executed in Iran in the early 1980s, would like Germany to take a stronger stance against the Iranian regime.

"In my opinion, Berlin is too cautious," Ahadi said. "We have to exert more public pressure on the Iranian regime. If we don't do or say something, people will die."