The international response to the crisis in Somalia has been inadequate, the UN's humanitarian affairs chief, John Holmes has said.
Aid agencies are having trouble operating because of the poor security situation, he said.
Fighting between Ethiopian-backed government forces and Islamist rebels in the capital, Mogadishu, has forced hundreds of thousands from their homes.
The World Food Programme (WFP) says it feeds 21,000 people a day in Mogadishu.
The WFP said it wants to bring that number up to 50,000.
Mr Holmes visited several camps for people displaced from the fighting in Mogadishu.
The UN refugee agency says 60% of Mogadishu residents have left their homes, including 200,000 this month, following the latest clashes between insurgents and the Ethiopian-backed government.
"This is obviously a very serious humanitarian situation in Somalia," he said.
Mr Holmes said the security situation was hampering relief efforts.
"There are checkpoints everywhere and aid agencies are stopped at these points and at times charged a lot of money."
He also met with Somalia's new Prime Minister Nur Hassan Hussein in Baidoa, where the interim government of President Abdullahi Yusuf Ahmed is based.
Mr Hussein has promised security and reconciliation will be his top priorities since being appointed last month.
"Reconciliation, security and provision of humanitarian aid are very much interlinked so we recognise this and definitely we will try to address them," Mr Hussein said.
A weak African Union force has been unable to stem the violence.
Only 1,600 Ugandan peacekeepers have arrived from a planned 8,000-strong force.
Somalia has not had a functioning national government since President Mohamed Siad Barre was overthrown in 1991.