The activities of rebels in northern Uganda are "terrorism of the worst kind anywhere in the world", UN humanitarian affairs chief Jan Egeland has said.
Security must be improved in the region where Lord's Resistance Army rebels abduct children and carry out attacks, he said while visiting Pader district.
Mr Egeland urged the Ugandan government and international community to do more to end the humanitarian crisis.
Almost two million people have been displaced during 20 years of civil war.
They live in camps, often in appalling conditions, in attempts to escape attacks by the LRA.
In addition, many thousands abandon their homes in rural villages every night for the relative safety of big towns.
In Pader district, the worst affected area of northern Uganda, Mr Egeland visited Patongo camp, a squalid home to about 40,000 people.
The BBC's Will Ross in Uganda said Mr Egeland did not hold back when assessing the situation, and described the current humanitarian relief effort as plasters on the wound.
"I don't think we really understand what it is when 90% of a population is terrorised into crammed camp conditions like this," Mr Egeland said.
"I just met a women's group where all of the women had had their children abducted, Most of them had never heard back from them."
Residents of the camps live in fear of rebel attacks
Some of the camp's residents told Mr Egeland about the problems they face, which included inadequate healthcare and poor access to education.
But the worst problem was insecurity. Mr Egeland said that although the rebels had become weaker, they remained strong enough to prevent people from returning to their villages.
"Everybody has to do more," he said. "The government of Uganda has to do more, the army has to provide real security for the people, not only when they are inside cramped camps but when they go out of these camps.
"We as aid organisations have to also improve conditions. Still too many are dying from lack of sanitation, lack of proper care."
Former LRA child fighters needed to be reintegrated and the rebel leaders brought to justice, he added.
As residents were telling Mr Egeland they did not feel it was safe enough to go home, the Ugandan military said it was clashing with pockets of rebels in the same district.
Our correspondent says the problem is how to end the war. Negotiations with the senior commanders seem to be out of the question as they have been indicted by the International Criminal Court, he adds.
Almost two million people have fled their homes
Mr Egeland said ending the insecurity was his hope for 2006. But he said he did not think there was a purely military solution.
He pointed to the insecurity caused by small groups of rebels and the fact the rebels were mainly abducted children.
"We are not wanting all the LRA killed - these are children, abducted children of these women around us.
"They should be able to demobilise and be reintegrated into society and I think it can happen," he told the BBC.
After meeting Ugandan President Yoweri Museveni on Friday, Mr Egeland said it was positive news the government of Uganda was acknowledging the situation more than it had previously, and was promising action.
On Sunday, the UN humanitarian affairs chief will visit southern Sudan, which is also blighted by Ugandan LRA attacks.