Mogadishu, Somalia (CNN) -- The United Nations is desperately trying to convince donors to get generous with their pocketbooks in order to stave off further suffering in the famine-wracked Horn of Africa.

The global body said almost $2.5 billion is needed to cope with the crisis. So far, it has only received 48% of that and is lacking $1.2 billion.

For Somalia, where the United Nations has declared famine in five southern regions, the United Nations has received about half of the roughly $1 billion needed.

"There's been an initial response but it needs to be sustained and we are still far short of the $1 billion target that we set ourselves with some very worrying gaps," said Mark Bowden, the U.N. coordinator for Somalia.

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Bowden said the United Nations will have to go back to donors and ask for more food. But the health sector is also an area of concern.

"What happens during famine is that people die of epidemic diseases like measles and malaria ... and if we can't support health activities then we will lose a lot of lives unnecessarily."

"We're worried that people are going to die in the next month or so and inevitably more areas will slip in to famine conditions," he said.

The situation is particularly grave in southern Somalia, where almost 3 million people are in need of immediate assistance.

In Mogadishu, U.N. agencies are assessing their ability to scale up operations after the militant Islamic group Al-Shabaab announced a withdrawal from the war-torn capital. But U.N. officials have said it is too early to gauge the consequences of the Al-Shabaab pullout.

Aid continues to reach Mogadishu by sea and air, but supplies are low.

In Al-Shabaab's absence, more Somalis have found the courage to defy the group's ban on foreign aid and seek refuge in government held areas.

Saacid, a local aid group that runs a children's feeding center, fed 18,000 children last month. This month, it's expecting to see 25,000 children.

At Banadiir Hospital, which houses Somalia's largest children's ward, the doctors work without electricity or running water. They are even running out of the saline drips used to rehydrate the malnourished.

Luwal Mohamed, the doctor in charge, said supplies are so scant that it's easier to count what the hospital has rather than what it doesn't.

"The supplies that are missing is the essential equipment for lifesaving like oxygen machines, monitoring, blood pressure measurements, stethoscopes," Mohamed said.

The United States announced Thursday an additional $17 million for the crisis, with $12 million designated for Somalia. The U.S. money is on top of the $105 million in emergency funding President Barack Obama announced Monday.

"What is happening in the Horn of Africa is the most severe humanitarian emergency in the world today and the worst that East Africa has seen in several decades," said U.S. Secretary of State Hillary Clinton.

Meanwhile, the Somali Football Federation lauded a $1 million donation from the governing body of international soccer.

"The FIFA humanitarian response shows that the world football governing body is not only confined to promoting football development in the lawless country, but also FIFA is playing a very big role in saving the vulnerable Somalis including some displaced by wars and draughts and some disabled by the long-exciting civil strife in the country," said a statement from the Somali federation.

The United Nations expects the Horn of Africa emergency to persist for several months and the number of people needing help to go up by 25%.

CNN's Nima Elbagir reported from Mogadishu and Moni Basu reported from Atlanta.