The International Committee of the Red Cross has appealed for news of three staff members abducted in Syria in 2013 - a New Zealand nurse and two Syrian drivers.
Nurse Louisa Akavi was snatched along with drivers Alaa Rajab and Nabil Bakdounes while travelling in a Red Cross convoy delivering supplies to Idlib, in the northwest of the country.
Armed men stopped their convoy on 13 October 2013, and abducted seven people, four of whom were released the following day.
The ICRC said it believed they were abducted by the so-called Islamic State group (IS).
"Our latest credible information indicates that Louisa was alive in late 2018," the group said in a statement from Geneva.
"The ICRC has never been able to learn more details about Alaa and Nabil, and their fate is not known."
New Zealand said it had dispatched a special forces unit to Syria to search for Ms Akavi, who was believed to be still held by IS.
"This has involved members of the NZDF (New Zealand defence force) drawn from the Special Operations Force, and personnel have visited Syria from time to time as required," said New Zealand Deputy Prime Minister Winston Peters.
"This non-combat team was specifically focused on locating Louisa and identifying opportunities to recover her."
Mr Peters said information about the kidnapping had not been previously released for fear that any publicity would place the hostages at greater risk.
"The past five and a half years have been an extremely difficult time for the families of our three abducted colleagues," said ICRC operations director Dominik Stillhart.
"Louisa is a true and compassionate humanitarian. Alaa and Nabil were committed colleagues and an integral part of our aid deliveries.
"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."
Ms Akavi had carried out 17 field missions with the ICRC and the New Zealand Red Cross, the statement said. Mr Rajab and Mr Bakdounes were "dedicated husbands and caring fathers", it added.
The war in Syria, which began in 2011, has claimed more than 370,000 lives and forced millions of people to flee their homes.
The Kurdish-led SDF, backed by a US-led coalition, captured the last IS bastion in eastern Syria on 23 March, and had detained thousands of suspected IS fighters.
But this could make it more difficult to find Ms Akavi.
The New York Times has reported the Red Cross has reason to believe she is alive, because at least two people described seeing her in December at a clinic in Sousa, one of the final villages to be held by IS jihadists.
"We are speaking out today to publicly honour and acknowledge Louisa's, Alaa's, and Nabil's hardship and suffering," the ICRC statement said.
The organisation has 98 foreign workers and 580 Syrians working in the country.
The Syrian Observatory for Human Rights accuses IS of abducting thousands of people since 2014.