The UN has admitted 15 Syrian children died after an NGO partner mistakenly gave them the wrong drug during a measles vaccination programme.
Stephane Dujarric, a spokesman for the UN secretary general, said on Wednesday that a muscle relaxant was given to the children and babies mid-September in rebel-held parts of northwest Syria.
"What happened with the vaccines was a real tragedy, but it was basic human error,'' Dujarric said.
A report by the World Health Organisation last month said the relaxant was kept in the same refridgerator as a substance used to dilute the measles vaccine. It said the exact person or group responsible for the laboratory was not known.
A doctor who witnessed the mistake, Abdullah Ajaj, told the AP news agency that those affected showed signs of "severe allergic shock", with many suffocating and suffering severe swelling.
Sujarric the incident had nothing to do with the quality of UN aid going into Syria.
Syria's ambassador to the UN, Bashar Ja'afari, last month called the deaths a crime against humanity and demanded the organisation investigate those responsible. .
After the children's deaths, the Western-backed opposition based in Turkey said it had suspended the second round of measles vaccinations. The campaign was meant to target 60,000 children.
Dujarric on Wednesday did not name the NGO partner and referred to the WHO report.
She defended the quality of UN in Syria, saying it had "seen no evidence of tainted food or medicine ... coming through UN channels."
Syria's health system has collapsed during three and a half years of war. Polio has emerged in some areas as nationwide vaccination efforts have failed.