Mark Lowcock said the situation echoed the tragedy in the country in 1984. That crisis inspired Bob Geldof to initiate the Band Aid superstar pop group to raise money to feed the starving.
The area most affected is the Tigray region, the northernmost regional state, where the national government is at war with the area’s leaders who once dominated Ethiopia’s government.
The economy has been destroyed, along with businesses, crops and farms, and there are no banking or telecommunications services, Mr Lowcock said. “We are hearing of starvation-related deaths already.”
Getting food and other humanitarian aid to all those in need was proving very difficult for aid agencies, he added.
Thousands of civilians and soldiers have been killed in fighting that began between forces from Ethiopia’s presidential government and the Tigray opposition in November.
Neighbouring Eritrea, which was part of Ethiopia until 1991, sided with Ethiopia’s central government, and many of the worst abuses in Tigray have been blamed on Eritrean soldiers.
Last week Mr Lowcock said an estimated two million people in Tigray had been displaced, and rape and other “abhorrent sexual violence” had been widespread and systematic. Hospitals and agricultural land had also been destroyed.
“There are now hundreds of thousands of people in northern Ethiopia in famine conditions,” Mr Lowcock said.
“That’s the worst famine problem the world has seen for a decade, since a quarter of a million Somalis lost their lives in the famine there in 2011. This now has horrible echoes of the colossal tragedy in Ethiopia in 1984.”
In the famine of 1984-85, about two million Africans died of starvation or famine-related ailments, about half of them in Ethiopia.
“The international community needs to really step up, including through the provision of money,” Mr Lowcock said, calling for G7 leaders to discuss the crisis.
The UN and the Ethiopian government have helped about two million people in recent months in government-controlled areas, he said.
But “there have been deliberate, repeated, sustained attempts” to prevent people in places controlled by Tigrayan opposition forces from getting food.
“Prime minister Abiy Ahmed needs to do what he said he was going to do and force the Eritreans to leave Ethiopia,” he said.
Additional reporting by AP