At least 15 children have died in South Sudan from a botched attempt to immunise them against measles, officials say.

The United Nations said the children died of “severe sepsis/toxicity” from the contaminated vaccine, and the health ministry blamed the deaths on human error. One syringe was used for all the children during the four-day campaign, and the vaccine was stored without refrigeration the entire time.

The team administering the vaccinations was “neither qualified nor trained” to do so, the South Sudanese health minister, Riek Gai Kok said.

The World Health Organization provides some training to South Sudan’s health officials and the U.N. children’s agency provides the vaccines to the government. It was not immediately clear whether any U.N. officials were present at the time of the botched vaccinations.

About 300 people were vaccinated during the campaign in the Kapoeta region, including 32 other children who fell ill, but survived, the health minister added.

Measles outbreaks in the country are a key target of the United Nations children’s fund (Unicef), which is aiming to vaccinate 1.2 million children this year.

But a report from the Associated Press said that in this case, children as young as 12 were administering the vaccine to others.

The government said all the children who died were under the age of 5. It is setting up a commission to determine who is responsible and whether victims’ families will be compensated.

Sepsis is a potentially deadly immune response triggered by an infection which spreads quickly in the body. It can lead to multiple organ failure and death.

South Sudan declared independence in 2011, but has struggled to maintain basic services – including healthcare – after a civil war erupted in 2013.

The United Nations estimates more than one million children have fled the country during the conflict, and a million more are displaced within the country.