Three Tibetans who have been on hunger strike outside the UN headquarters for the past month ended their protest Thursday after the UN said investigators would look into events in Tibet, a protest organizer said.

One of the three was in hospital when two UN officials handed over a letter to the other two from UN human rights chief Navi Pillay. They struggled out of wheelchairs to sing the Tibetan anthem after the protest was ended.

The letter from Pillay said that the rights chief had "assigned special rapporteurs of the United Nations to look into the situation inside Tibet," protest organizer Tsewang Rigzin told AFP.

"Navi Pillay herself has an open invitation to China and they are working on getting the date fixed," added the official from the Tibetan Youth Congress.

Rigzin said that as the United Nations was trying to assess the situation in Tibet and the two officials had personally handed over the letter to the hunger strikers they had decided to call off the protest.

"This is a small victory," he said.

Dorjee Gyalpo, Yeshi Tenzing and Shingza Rimpoche started refusing food and sat outside the UN headquarters on February 22. They had vowed to fast until a UN fact-finding mission was sent to Tibet and also demanded international pressure on China to end what they call an "undeclared martial law" in Tibet.

New York police would not let them sleep in the street and each day made the three prove they could stand up. Police made Shingza Rimpoche, 69, go to hospital on Monday, but he had continued to refuse food.

UN leader Ban Ki-moon had expressed concern about the health of the three on March 12 through his spokesman.

On Thursday, Richard Bennett, an advisor to an assistant secretary general for human rights and an official from Ban's office, Parfait Onanga, handed over the letter to the Tibetan protesters.

"The letter is addressed to the three hunger strikers, and they have agreed to end their fast," Bennett said afterwards. "We brought them juice to break their fast."

"I think everyone has the right to peaceful protest. But we are also relieved that this particular protest has concluded," Bennett added. He would not reveal the contents of the letter.

China is extremely sensitive to criticism on Tibet, where there have been a growing number of self-immolation protests in recent months. A 20-year-old Tibetan Buddhist monk has died in detention after he set himself on fire in a town in southwest China, a US-based rights group said Wednesday.

Many Tibetans in China complain of religious repression, as well as the gradual erosion of their culture.

China put down a 1959 uprising led by the Dalai Lama, who is currently in exile. It says Tibetans now lead better lives than ever thanks to huge government investment.