Ethiopia's government has increased to 10.1 million the estimated number of people who desperately need food aid because of a drought.
More than half of them are children, Save the Children says.
The drought, blamed on the El Nino weather phenomenon caused by Pacific Ocean warming, was the worst in 50 years, the charity added.
In November, the UN children's charity put the number of Ethiopians threatened by hunger at 8.2 million.
The government has launched a huge national effort, allocating nearly $200m (£130m) to deal with the food crisis, says BBC Africa correspondent Alastair Leithead.
But with 10.1 million people - or a tenth of the population - affected by a failure of both harvests this year in parts of Ethiopia, more international aid would be needed, he adds.
Confirming the figure in a BBC Focus on Africa radio interview, Getachew Reda, a government minister and aide to the prime minister, said: "We are trying to make sure that no-one is affected to such an extent that they lose their lives."
Save the Children said an "emergency response" to deal with the crisis could cost $1.4bn.
"An estimated 400,000 children are now also at risk of developing severe acute malnutrition in 2016, which can lead to stunting, and physical and mental delays in development," it said in a statement.
Several other African states - including Malawi and South Africa - have also been hit by drought.
A famine in Ethiopia in 1984 led to hundreds of thousands of people starving to death.
However, the Ethiopian Embassy in London, in a statement last month, said the country was not at risk of a "famine of any sort, let alone anything remotely like the magnitude of that of 1984".
Ethiopia was also hit by a drought in 2011 and 2008, causing a food crisis.