(CNN) -- An Iranian woman convicted of murder -- in a killing that human rights groups called self-defense against a rapist -- was hanged Saturday, state news agency IRNA reported.
Reyhaneh Jabbari, 26, was sentenced to death for the 2007 killing of Morteza Abdolali Sarbandi, a former employee of Iran's Ministry of Intelligence and Security.
The United Nations has said she never received a fair trial. The U.S. State Department also said there were concerns about the trial.
"There were serious concerns with the fairness of the trial and the circumstances surrounding this case, including reports of confessions made under severe duress," State Department spokeswoman Jen Psaki said Saturday.
"We condemn this morning's execution in Iran of Reyhaneh Jabbari, an Iranian woman convicted of killing a man she said she stabbed in self-defense during a sexual assault," Psaki said.
Jabbari's execution was originally scheduled for September 30, but was postponed. Amnesty International said the delay may have been in response to the public outcry against the execution.
Jabbari was convicted of murder after "a flawed investigation and unfair trial," according to Amnesty International.
The United Nations has said Sarbandi hired Jabbari -- then a 19-year-old interior designer -- to work on his office. She stabbed him after he sexually assaulted her, it said.
Jabbari was held in solitary confinement without access to her lawyer and family for two months, Amnesty International said in a statement. She was tortured during that time, the group said.
"Amnesty International understands that, at the outset of the investigation, Reyhaneh Jabbari admitted to stabbing the man once in the back, but claimed she had done so after he had tried to sexually abuse her," the rights group said. "She also maintained that a third person in the house had been involved in the killing. These claims, if proven, could exonerate her but are believed never to have been properly investigated, raising many questions about the circumstances of the killing."
Iranian Oscar-winning director Asghar Farhadi joined scores of Iranian artists and musicians calling for a halt to the execution. In an open letter, Farhadi asked the victim's family to pardon her, a possibility under Iranian law.
Rights groups have criticized Iran for a surge in executions under Hassan Rouhani in his first year as president.
UK Foreign Office Minister for the Middle East Tobias Ellwood said he was "very concerned and saddened" that Jabbari had been executed, especially given the questions concerning due process in the case.
"The UN Special Rapporteur for Human Rights in Iran, Dr Ahmed Shaheed, noted that her conviction was allegedly based on confessions made while under threat, and the court failed to take into account all evidence into its judgment," he said in a statement.
"Actions like these do not help Iran build confidence or trust with the international community. I urge Iran to put a moratorium on all executions."
According to the United Nations, Iran has executed at least 170 people this year. Last year, it executed more people than any other country with the exception of China, the world's most populous nation.
U.N. Secretary-General Ban Ki-moon, speaking in July, said the death penalty has no place in the 21st century and urged all countries to work toward its abolition.
CNN's Ben Brumfield and Reza Sayah contributed to this report.