A New Zealand nurse was held hostage by Islamic State for almost six years but her identity was kept under wraps until today because of fears it would put her life in danger.

The nurse, Louisa Akavi, was named in the New York Times today with confirmation from the International Committee of the Red Cross.

It effectively ended a five and a half year agreement by media around the world to not name her, or her nationality, because of concerns held by the New Zealand Government that she would be killed by her captors.

Akavi's fate and whereabouts are unknown.

Islamic State territory was wiped out with the fall of Baghouz last month, but security forces have not yet been able to find Akavi or get confirmation of whether she was still alive.

It is possible she is among the thousands in camps but has not identified herself or is still a captive being held elsewhere.

Akavi, 62, was working for the Red Cross when taken hostage in Syria in October 2013 along with five other Red Cross workers and one Red Crescent.

Since then, she has been held captive by Islamic State in an ordeal believed to have seen her offered for ransom and eventually serving as a human shield. Her medical skills in tending to wounded Islamic State fighters and leaders may also have offered her some protection from the fate suffered by several other hostages.

Akavi is a Cook Island New Zealander who owns a home in Otaki and has family in Porirua. She has worked for years as a nurse in some of the world's most dangerous places.

Today's publication of her identity is understood to be against the wishes of New Zealand authorities, who remained concerned about identifying her while there was still any chance she was being held by Islamic State members.

In the past, the New York Times was among the media which agreed to withhold details about Akavi.

In a statement, Dominik Stillhart, director of operations for the ICRC, said the organisation had decided to go public with Akavi's case because it hoped it would lead to information on her whereabouts after the fall of Baghouz, in eastern Syria.

"We have not spoken publicly before today because from the moment Louisa and the others were kidnapped, every decision we made was to maximise the chances of winning their freedom.

"With that goal in mind, we have long decided not to share details in the hopes this approach would lead to a positive result. With Islamic State group (ISg) having lost the last of its territory, we felt it was now time to speak out.

"Following the fall of the last ISg stronghold in Syria, we hope there will be new opportunities for us to learn more about Louisa's situation. But we also fear there is an extra risk of losing track of her in the aftermath.

"That is why we are calling for any information that could provide more leads into her whereabouts and wellbeing. We remind everyone that she is a victim of a kidnapping, and a hostage who has been held for many years."

Of the seven hostages taken, four were released soon after. As well as Akavi, aid workers and Syrian nationals Alaa Rajab and Nabil Bakdounes were still missing and nothing had been heard of them.

Stillhart said the last credible sighting of Akavi was in late 2018. He also outlined efforts to try to get Akavi released, including negotiating with Islamic State in late 2013 and early 2014, but none were successful.

"The past five and a half years have been an extremely difficult time for the families of our three abducted colleagues. Louisa is a true and compassionate humanitarian. Alaa and Nabil were committed colleagues and an integral part of our aid deliveries," said Stillhart.

"We call on anyone with information to please come forward. If our colleagues are still being held, we call for their immediate and unconditional release.

"We are speaking out today to publicly honour and acknowledge Louisa's, Alaa's, and Nabil's hardship and suffering. We also want our three colleagues to know that we've always continued to search for them and we are still trying our hardest to find them. We are looking forward to the day we can see them again," Stillhart said.

The statement also set out some of what was known about Akavi's actions over that time, taken from people interviewed in camps in former Islamic State territories. It included providing medical care for some of her captors.

Akavi is the longest held hostage in the history of the International Red Cross. The Red Cross also paid tribute to her work.

While several other hostages have been named, the New Zealand Government and diplomats managed to secure agreements from both domestic and international media not to name Akavi or state her nationality. They were backed by the ICRC.

That was because Islamic State had warned her family she would be killed if news of her capture became public.

New Zealand officials were also aware Akavi may have taken the precaution of identifying herself as a Cook Islander, rather than a New Zealander, to her captors as a security measure.

A highly-placed source, who has seen intelligence briefings relating to the hostages, said: "She was being held... is being held by the same people who have been responsible for the executions."

Her situation was described as "pretty grim" and her chance of release "slim".

She had also suffered illness through the long period of captivity, according to statements from those who were held with her but had since escaped.

It is understood one of the first interactions between Akavi and her Islamic State captors was to obtain an email address through which contact was made with her family in New Zealand.

The Herald has been told the captors then used that to demand a ransom and warned media coverage of her situation would lead to her death.

Her family approached the government, which the Herald understands formed the view she could be killed if her nationality was identified.

As a result, her awful situation became a closely held secret.

Sporadic negotiations over the offered ransom began between the International Committee of the Red Cross and her captors but went nowhere, the Herald has learned.

The Herald has been told that she was known to be held with - until at least September 2014 - American aid worker Kayla Mueller, who Islamic State reported killed by an air strike in 2015.

In October that year, the New Zealand government received "proof of life" information about Akavi.

Nothing was heard of the Kiwi nurse from then until her presence was raised by Islamic State as a bargaining chip to escape Baghouz in February.

Those trapped in the doomed town had sought to buy their freedom by offering the names of hostages as a bargaining chip to be allowed free passage.

They included Akavi, British journalist John Cantlie and an Italian priest, Paolo Dall'Oglio.

Akavi's desperate situation was a constant focus for New Zealand intelligence services and the Ministry of Foreign Affairs and Trade throughout her captivity.

When the new government came into power in 2017, her plight was the focus of early briefings to Prime Minister Jacinda Ardern.

The briefings were Top Secret and given a high priority to impress on the new administration the seriousness of the situation.

The awful plight of Akavi was understood to have been part of considerations by Prime Minister John Key when deciding on sending troops to Iraq.

The Herald has been told Key considered the potentially fatal impact the deployment could have before deciding her plight could not dictate or impact on New Zealand's response.

The Herald learned of Akavi's captivity in 2014. Inquiries in January sparked a call from then Foreign Affairs minister Murray McCully, who explained the risk to the woman and urged the Herald to hold off reporting. An editorial decision was made to hold off on the story until she was identified by her captors, or the situation was resolved.

Other media also learned of Akavi's captivity and also agreed to hold off reporting, including international media.

Negotiations with media included the US ambassador in Washington making contact with the New York Times, having learned it was intending to identify the woman by her nationality in a story about hostages.

The Herald has confirmed the diplomatic plea was successful with the newspaper pulling the identity from its print and online reporting.

Further diplomatic efforts were required last month as news of Islamic State's offer became known among some around Baghouz. Again, New Zealand diplomats were able to appeal to editors - including Britain's Sunday Times - to withhold her identity and even nationality, for fear it would lead to her death.

Ransoms of hostages has proved lucrative for ISIS, with a United Nations terrorism estimating in November that the terror group had earned $45-60m in ransoms in the previous 12 months.

New Zealand, like Five Eyes partners the United States and United Kingdom, has a policy of not paying ransoms. Some European governments, including France, Germany and Spain, have paid ransoms for the return of citizens.

US special forces attempted a rescue of the group in July 2014. They found they were too late and the hostages had been moved.

No hostages had been ransomed since August. It was about this time ISIS embarked on an horrific social media campaign using the deaths of hostages believed to have been held with Akavi.

The first to be killed by "Jihadi John" was American journalist James Foley, who was beheaded in August. Others in the group of captives were believed to be American emergency medical technician Peter Kassig and aid worker Alan Henning, who were also beheaded.