Image caption His family received this image of Levinson in 2011

An American ex-FBI agent believed to have been held in Iran for the last seven years was working for the CIA on an unapproved mission, the Associated Press news agency reports.

Robert Levinson disappeared during a business trip to the Iranian island of Kish in March 2007.

US government officials have repeatedly called on Iran to help locate him.

"We have no comment on any purported affiliation between Mr Levinson and the US government," said a CIA statement.

"The US government remains committed to bringing him home safely to his family," added CIA media spokesperson Todd Ebitz.

The Associated Press report says that the CIA paid off Mr Levinson's family and reprimanded several analysts involved.

The new agency reported that Mr Levinson was in Iran on an unapproved intelligence-gathering mission that, when revealed, caused serious tumult within the CIA.

The US agency reportedly paid Mr Levinson's family $2.5m (£1.5m) to avoid a public lawsuit, and also disciplined 10 veteran analysts.

The team of analysts is said to have paid Mr Levinson to gather intelligence prior to his disappearance.

Three of the CIA analysts who hired him - who reportedly had no authority to run spy operations - were allegedly later forced out of the agency.

All efforts fail

We respectfully ask the government of the Islamic Republic of Iran to assist us in securing Mr Levinson's health, welfare, and safe return. Jay Carney, White House spokesman

The Associated Press reporters interviewed senior US and foreign officials and reviewed confidential documents.

The media outlet says it was asked by the US government three times since 2010 to hold the story about Mr Levinson's CIA ties.

The news agency said it decided to run the report after all efforts to locate and free Mr Levinson seemed to have failed.

"The US government strongly urged the AP not to run this story out of concern for Mr Levinson's life," said National Security Council spokesperson Caitlin Hayden.

"We regret that the AP would choose to run a story that does nothing to further the cause of bringing him home."

Neither the identities of the former FBI agent's captors nor his whereabouts have been confirmed, but US officials asked for Iran's assistance in finding him just days after Iran and Western powers signed an interim agreement aimed at curbing Iran's nuclear programme in late November.

"We reiterate the commitment of the United States Government to locate Mr Levinson and bring him home safely to his family, friends, and loved ones," White House spokesman Jay Carney wrote in a statement at the time.

"We respectfully ask the government of the Islamic Republic of Iran to assist us in securing Mr Levinson's health, welfare, and safe return."

In August, US Secretary of State John Kerry also asked for Iran's help in freeing Mr Levinson and two other US citizens held in Iran, a former marine and a Christian pastor.

'Help me'

Image caption The FBI released a composite image of Robert Levinson and offered a reward for information leading to his return on 6 March 2012

Mr Levinson, 64 when he went missing, was initially said to have been investigating cigarette counterfeiting as a private detective when he disappeared.

His family received images of Mr Levinson in April 2011, showing him wearing a long grey beard and an orange jumpsuit, holding up five signs that read:

4th YEAR... You can't or you don't want...?

This is the result of 30 years serving for USA

Why you can not help me

I am here in Guantanamo - Do you know where it is?

Help me

The family were also sent a video in November 2010, which it released in December 2011 to try to aid the investigation.

In the 54-second clip, Mr Levinson pleads: "Help me get home."

On Thursday, Mr Levinson's family said the US government had not done all it could to rescue him.

"The US government has failed to make saving this good man's life the priority it should be," Mr Levinson's family wrote in a statement provided to the BBC Persian service.

"There are those in the US government who have done their duty in their efforts to find Bob, but there are those who have not. It is time for the US government to step up and take care of one of its own. After nearly 7 years, our family should not be struggling to get through each day without this wonderful, caring, man that we love so much."

Iran has said it does not know where Mr Levinson is and that there is no evidence he is in the country.

Investigators traced the phone used to send the photographs to Afghanistan, but the phone's owner was not involved. The video was sent from a Pakistan internet cafe.

The FBI offered a $1m reward in March 2012 for information leading to Mr Levinson's safe return.

But the US government has not received any sign Mr Levinson is alive in nearly three years, the Associated Press reports.