Only about 9% of health facilities in Tigray are fully functional amid access constraints and fuel shortages, WHO officials told journalists in Geneva.
Those that can still operate are resorting to using saline solutions to treat wounds and rags to dress them, they said.
"In these situation of hardship and limited access, often death happens at a community level that goes underreported and unregistered," said Altaf Musani, WHO Director of Health Emergencies Interventions, describing the situation as "deeply worrying."
The conflict that has pitted Ethiopia's army against forces from the country's northern region of Tigray has has led to a de facto blockade that has lasted around two years, although some aid supplies reached communities between March and August during a temporary ceasefire which has since been broken.
The fighting has killed thousands, displaced millions and left thousands on the brink of famine.
Spokespeople for the prime minister, health minister and a government spokesperson did not immediately respond to a request for comment. The Tigray forces' spokesperson Getachew Reda did not immediately respond either.
Meanwhile, Ethiopian and Tigrayan officials are meeting in Pretoria, South Africa, to try to find peace between them. Those discussions, as presently scheduled, run until Sunday.
