UN Secretary General Ban Ki-moon announced the immediate despatch of the humanitarian team at a news conference in Brussels on Thursday.
Open the gates of hell and allow these people out into safety
UN Refugee Council
"So many lives have been sacrificed. There is no time to lose," Mr Ban said.
He said the new team would monitor the situation and the UN would do "whatever we can to protect the civilian population who are caught in the war zone."
Hours later, the UN Refugee Council's representative in Colombo, Amin Awad, called on the Sri Lankan government to allow pauses in the fighting so the necessary work could be completed.
"A rapid humanitarian assessment and delivery of much-needed supplies is needed and needed now," he told the BBC.
"We are calling on the government to restrain itself and have the moral upper ground by allowing the humanitarian aid in, and we're asking the LTTE [Tamil Tigers] to open the gates of hell and allow these people out into safety."
India has also expressed concern about stranded civilians and senior ministers are travelling to Colombo to raise the humanitarian issue with the Sri Lankan government.
Doctors say many people have been treated for gunshot and blast injuries
India's Foreign Minister, Pranab Mukherjee, called for an end to "the continued killing of innocent Tamil civilians in Sri Lanka".
"The Sri Lankan government has a responsibility to protect its own citizens and the LTTE must stop its barbaric attempt to hold civilians hostage," said Mr Mukherjee.
"There is no military solution to this ongoing humanitarian crisis, and all concerned should recognise this fact."
Thousands of people are caught with the Tamil Tigers in a 12 sq km (5 sq m) area in the north of the country as the military closes in.
The UN's humanitarian coordinator, Neil Buhne, said tens of thousands of people were living in camps in the northern town of Vavuniya.
We are doing emergency surgery, but the hospital is completely overwhelmed
Dr Paul McMaster, MSF
"I saw infants with dysentery, malnourished children and women, untended wounds, and people dressed in the ragged clothing they've been wearing for months," the Associated Press quoted him as saying.
Paul McMaster, a British surgeon with Medicins Sans Frontieres (MSF), told the BBC a "continuous stream" of patients had been arriving at the hospital in Vavuniya since the weekend.
He said the hospital was equipped with 400 beds but was treating nearly 2,000 patients, many of them with gunshot wounds and blast injuries.
"We are doing emergency surgery, but the hospital is completely overwhelmed," he said, with patients lying on the floor, in corridors and outside under trees and temporary shelters.
On Thursday, Sri Lankan Defence Secretary Gotabhaya Rajapaksa rejected a UN appeal to allow aid agencies in.
View satellite images showing area in northern Sri Lanka where refugees have gathered on the beach.
"It's not a sensible thing at the moment," he told the BBC.
"There is a civilian rescue operation going on in the area and allowing aid agencies inside the conflict zone is not matching with ground realities."
While the government has allowed aid agencies to help those fleeing the conflict, Sri Lanka's UN ambassador says only the International Committee of the Red Cross (ICRC) and the Catholic charity, Caritas, have been let into the war zone itself.
The ICRC said they had evacuated 530 people on Thursday and 350 the day before, including families as well as sick and wounded people.
On Wednesday the UN Security Council, which had been accused of inaction, called on the Tamil Tigers to lay down their arms and urged the Sri Lankan government to allow international aid agencies into areas of need.
The UN and other Western nations - including the US and the UK - have been pressing for an immediate halt to the fighting to allow time for civilians to leave the war zone safely.
The government categorically denies rebel charges that hundreds of civilians have been killed by the army, saying soldiers are only lightly armed and are trying to rescue trapped people.
The Tamil Tigers, meanwhile, deny accusations that they are holding civilians as human shields.
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