David Cameron has said other EU countries must do more to aid the famine-hit Horn of Africa.
The UK prime minister said Britain had spent £90m - but it was time other countries "stepped up to the plate".
Labour leader Ed Miliband also called on other countries, including the US, to "do a far better job" .
The UN said parts of Somalia were suffering a famine after the worst drought in 60 years, also affecting Ethiopia, Kenya, and South Sudan.
Some 10 million people are said to need food aid across East Africa but Somalia is by far the worst-affected country.
Speaking after an hour long meeting with members of the Somali community in Birmingham, Mr Cameron said: "Britain is showing very strong leadership.
"We have done the most and my aid minister has been to the Horn of Africa to see for himself what needs to be done.
"We have three-and-a-half million people affected by famine and two million of those are not getting any help.
"It's time for other European countries to start doing what Britain is doing, and to give more to make sure the aid gets through and we save lives."
Mr Cameron acknowledged long-term solutions were required in Somalia, to strengthen its government but stressed that more aid was urgently needed to prevent people dying.
"We have given a large amount of money - £90m, more than other countries.
"It's time, frankly, for other countries to step up to the plate and recognise what's happening," added Mr Cameron.
Mr Miliband - speaking on a visit to the Disasters Emergency Committee offices in London - said the British public "are to be thanked" for the £27m they have donated to the East Africa Crisis Appeal.
But the Opposition leader added: "There is a huge humanitarian emergency in Somalia and Ethiopia. That's why we've got to get European governments to start stumping up the money, because so far they haven't.
"The British government is doing a good job on this, but the other European governments, the United States, need to do a far better job, because half a billion pounds is needed."
He added: "We've got 10 million people at the moment who are in very difficult circumstances, facing the terrible drought, and we need those governments as well as the public to now show that they are going to give the money that is required."
DEC Chief Executive Brendan Gormley said the British public had been "stunningly generous", but added: "The gap between what is needed and what can be paid for is still sadly enormous."
The money donated would go to the "absolute basics of life" including clean water, food and shelter, he added.
It comes as aid charity Unicef prepared to airlift a further 105 tonnes of food and medical supplies to the region.
So far 1,300 tonnes of emergency supplies have been sent to the area.
Unicef's UK executive director David Bull said: "We have stepped up our response even more as the death rate among severely malnourished children under the age of five in Lower Shabelle in south Somalia has climbed dramatically.
"They are now dying at a rate of more than 250 per day - that's one child every six minutes. This is totally unacceptable and we must all act now to help save lives."
The charity estimates it will need £62.8m over the next six months to help children affected by the drought.