UN Secretary General Ban Ki-moon has urged Sri Lanka to allow "unhindered access" by aid agencies to displaced people in the war-ravaged north.
He spoke after visiting a huge camp for refugees who fled fighting between Tamil rebels and government forces.
Following talks with President Mahinda Rajapaksa, Mr Ban said the government was "doing its utmost" but that more could be done to assist victims.
Earlier this week the government said it had defeated the 26-year insurgency.
'Humbled'
On Saturday, the UN secretary general saw first-hand the main government-run camp for about 220,000 refugees at Manik Farm, near Vavuniya.
"I was humbled by what I saw," he told reporters afterwards.
Humanitarian agencies say access to hundreds of thousands of refugees has been restricted and the distribution of aid hampered by a ban on vehicles from the UN and other groups.
"It was a very sobering visit, very sad and very moving"The government says it needs more time to find any Tamil Tiger guerrillas hiding in the camps, and is suspicious of some agencies which it has accused of helping the rebels.
It plans to resettle most refugees with six months.
"We will try to work hard to keep that promise realised," Mr Ban said. "They need to be resettled as soon as possible."
After talks with President Rajapaksa at his residence in Kandy, Mr Ban said: "The government is doing its utmost best", but added that there was a "wide gap between what is needed and what can be done".
Sri Lanka has appealed for $151m in UN funds to improve the camps.
Mr Ban also called for political reconciliation between the majority Sinhalese and minorities, including Tamils.
"If not history could repeat itself", he said.
Without a political settlement that gives Tamils real rights, UN officials believe the fighting will begin anew, says the BBC's Laura Trevelyan, who is travelling with Mr Ban.
Battleground
On Saturday, Mr Ban also took a low-level helicopter flight over the coastal area where the final battle was fought.
"It was a very sobering visit, very sad and very moving," he said of the scene of the battle.
Sri Lanka officially announced an end to the war this week, after its troops took the last segment of land held by the rebels and killed the Tamil Tiger leadership, including its chief, Velupillai Prabhakaran.
It is thought at least 80,000 people were killed in the war.
The UN says 7,000 civilians have died since January alone, although the government disputes this figure.
At a rally before Mr Ban arrived, Mr Rajapaksa dismissed any attempt to take him to an international war crimes court.
"There are some who tried to stop our military campaign by threatening to haul us before war crimes tribunals.
"I am not afraid. The strength I have is your support. I am even ready to go to the gallows on your behalf."
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