Iranian Nobel laureate Shirin Ebadi has said Iran's judiciary is preventing her from representing a US-Iranian academic detained in Tehran last week.
Haleh Esfandiari is one of the leading US experts on Iran
Haleh Esfandiari is being held at Evin prison while she is investigated for crimes against national security.
Mrs Esfandiari had been trapped in Iran since December, when her passports were stolen by masked men wielding knives.
Ms Ebadi, a lawyer, said she had requested representation in a phone call, but Iran's judiciary denied it.
As a result, she has not been allowed to see her client and has not been given details of the case.
Earlier this week the US Secretary of State, Condoleezza Rice, called for Mrs Esfandiari's immediate release.
But Iran's foreign minister described the academic, who works at the Woodrow Wilson Centre in Washington, as an Iranian national and said Washington should avoid meddling in the affairs of other countries.
Ms Ebadi, who was awarded the Nobel Peace Prize in 2003, told the BBC she did not believe Iran had a right to take this position.
"In my view, human rights are an international issue and all countries and all people have the right to comment on human rights issues anywhere in the world," she said.
"In the same way that we have the right to talk about the situation in Palestine, the world has the right to comment on Iran's situation."
The Iranian media have alleged that the 67-year-old was spying for Israel, or had converted to Judaism - a charge punishable by death.
Such allegations have been strongly denied by her family, who say she was in Iran to visit her ailing 93-year-old mother and is not guilty of any crime.
Ms Ebadi said she would pursue all available channels to help her client.
"I will use all legal methods to defend my client and to prove her innocence and I hope that this can be carried out in Iran's courts," she said.
"However, if it is not possible to do this in Iran¿ I will use all other methods that are available, including seeking help from international organisations."
Earlier this week, Mrs Esfandiari's husband told the BBC his wife's detention could be due to what he described as excessive zeal on the part of Iranian intelligence agents, who seemed to feel exchanges between Iranians and Americans were somehow sinister and destabilising.