The World Health Organisation chief said urgent action was needed to avert “genocide” in Ethiopia's Tigray region as the international community increased pressure for peace talks.
Fighting intensified after a ceasefire broke down in August, and is taking an “utterly staggering” toll on civilians, the UN said.
Tedros Adhanom Ghebreyesus told reporters from WHO headquarters in Geneva that there is a “very narrow window now to prevent genocide in Tigray.”
Dr Tedros, who is from the region, said the people of Tigray had essentially been blocked off from essential services since the conflict began in November 2020.
“There is no other situation globally in which six million people are being kept under siege for almost two years,” he said.
“Banking, food, electricity and healthcare are being used as weapons of war,” he said.
Troops from Ethiopia and neighbouring Eritrea have been waging an offensive near Shire for several days, and claimed on Tuesday to have captured three towns in Tigray.
Shire, home to about 100,000 people before the conflict began, is about 300 kilometres by road north-west of Tigray's capital Mekele. Korem and Alamata lie about 180km south of Mekele.
It is not possible to verify claims from either side as Tigray is under a communications blackout and access to northern Ethiopia is restricted.
Dr Tedros' comments joined a chorus of calls for peace efforts between the Tigrayan rebels and the government of Prime Minister Abiy Ahmed, who have been at war since November 2020. A five-month ceasefire ended in August.
On Monday, UN Secretary General Antonio Guterres said that the situation was “spiralling out of control”.
“Violence and destruction have reached alarming levels,” he said, calling for the “immediate withdrawal and disengagement” of Eritrean forces.
The EU, the US and the African Union have also issued urgent appeals for a halt to the fighting, which is threatening the stability of the continent's second most populous nation and the wider Horn of Africa region. An attempt by the AU to mediate talks this month failed.
“Even people who have money are starving because they can't access their bank for two years,” he said, adding that “children are dying every day from malnutrition.”
Dr Tedros acknowledged that he is personally affected by the situation in Tigray.
“Most of my relatives are in the most affected areas,” he said, but insisted that “my job is to draw the world's attention to crises that threaten the health of people wherever they are”.