Cases have increased by 80 percent compared to one year ago, said Ilham Abdelhai Nour, the WHO's head of emergency operations for Ethiopia.
"We need to implement and undertake activities to prevent and treat malaria," she told a press briefing in Geneva, but the WHO has had no air or road access to Tigray for the last six weeks.
For the first time since the 1960s, people in Tigray do not have access to preventive interventions against malaria, such as prophylactic drugs, according to the UN health agency.
The WHO had been able to transport in some material from March to August during the humanitarian truce, but the lack of fuel has severely limited the resupply of health centers in the region.
WHO director-general Tedros Adhanom Ghebreyesus, who is himself from Tigray, said the six million people of Tigray had been "kept under siege for almost two years."
Altaf Musani, the WHO's director of health emergencies interventions, called for sustained access to all parts of Ethiopia.
"When you look at the range of health risks, it's large, it's immediate, it's real," he said Friday.