Ms. Ashtiani's case has not proceeded with the transparency and due process guaranteed under Iranian law, and we are concerned about reports of coerced confessions and other mistreatment 

U.S. State Secretary Hillary Clinton

"Ms. Ashtiani's case has not proceeded with the transparency and due process guaranteed under Iranian law, and we are concerned about reports of coerced confessions and other mistreatment," Clinton said.

"The United States joins the international community in calling for Iran to immediately halt any plans for Ms. Ashtiani's execution and to handle her case with utmost transparency," she said.

A French human rights group and exiled Iranian activist Mina Ahadi have voiced fear that Ashtiani could be executed as early as Wednesday.

The case drew an international outcry after it emerged she would be stoned to death.

Since July, Iran has repeatedly said that the stoning sentence has been stayed pending a final decision and it was unclear how any execution would be carried out.

The United States is the only major Western nation to practice the death penalty, but has said in response to Ashtiani's case that it does not believe adultery should be a capital offense.

British Foreign Office Minister Alistair Burt spoke to Iran's charge d'affaires in London, Safar Ali Eslamian Koupaei, by phone on Tuesday to press for an update on Ashtiani's case.

But the Iranian diplomat was unable to confirm whether reports about her imminent execution were accurate, the Foreign Office said.

"I ... took the opportunity to remind him that the UK government would regard the execution of Ms Ashtiani as utterly unacceptable," Burt said in a statement.

 I ... took the opportunity to remind him that the UK government would regard the execution of Ms Ashtiani as utterly unacceptable 

British Foreign Office Minister Alistair Burt

A government spokesman said in September Ashtiani's adultery conviction was under review, but the charge of being complicit in the murder of her husband was still pending.

Under the Islamic law in force in Iran since the 1979 Islamic Revolution, murder is punishable by hanging, adultery by stoning.

President Mahmoud Ahmadinejad fended off questions about the case from reporters when he attended the U.N. General Assembly in September, saying it had been fabricated by hostile Western media and called the United States hypocritical for its record on executions.

The case has worsened relations between Iran and the West, which are locked in a dispute over Tehran's nuclear program, and was further complicated last month when two Germans were arrested in Iran while conducting an interview with Ashtiani's son.

The men entered Iran with tourist visas and were not authorized to act as journalists, judicial officials said. The German government is trying to secure their release.

In August, Iranian television aired an interview with a woman it said was Ashtiani admitting a relationship with a man who then murdered her husband. The International Committee Against Stoning, called the TV show "toxic propaganda."

According to Amnesty International, Iran is second only to China in the number of executions it carries out. It put to death at least 346 people in 2008.