International Development Secretary Andrew Mitchell has condemned the "derisory" response by some countries to the crisis in Somalia after a state of famine was officially declared.
Mr Mitchell said Britain is "playing its part" by supplying aid to the Horn of Africa and "others must do the same".
Tens of thousands of people have died in war-torn Somalia due to malnutrition amid the worst drought for over half a century.
The UN said that two regions, Bakool and Lower Shabelle, are now suffering famine under its formal definition. That means at least two adults or four children per group of 10,000 people are dying of hunger every day and 30% of the population is acutely malnourished.
Mr Mitchell, who recently visited aid camps, said: "In Somalia, men, women and children are dying of starvation. The fact that a famine has been declared shows just how grave the situation has become.
"It is time for the world to help but sadly the response from many countries has been derisory and dangerously inadequate.
"Britain is playing its part, with help for more than two million people across the Horn of Africa. Now others must do the same."
Shadow international development secretary Harriet Harman said: "It is deeply distressing that the situation in the Horn of Africa is deteriorating so rapidly that the UN has announced famine in the region for the first time in 19 years.
"Some 3.7 million people, almost half the country's population, are in urgent need of humanitarian assistance with one in three children suffering from malnutrition.
"The world must step up action to tackle this tragedy which is unfolding before us. Urgent international action is critical in order to prevent the spread of famine and further loss of life. So far, the international response has been slow and inadequate."
