About 300,000 people have fled their homes, while another 75,000-100,000 people - not counting children - are thought to have taken refuge in Uzbekistan.
The Red Cross (ICRC) has described the situation as an "immense crisis".
Some aid has begun to arrive in the region, but the ICRC says refugees are running short of basic supplies.
At least 40,000 refugees are without shelter.
People have been leaving after violence between Kyrgyz and ethnic Uzbeks last week, in which at least 191 were killed. Some observers have said the death toll is higher.
The latest UN Humanitarian Office figure exceeds previous estimates of 250,000-275,000 displaced.
Over the border in Uzbekistan, many of the displaced - mostly women and children - are in makeshift camps.
Many report instances of rape, and severe beatings.
"We need clothes and medical supplies, especially for the children, because when we fled our homes we just ran away and couldn't take anything with us," said Halima Otajonova, a 41-year-old mother of two, at a refugee centre at a stadium in the Uzbek town of Khanabad.
"Some of us even ran away in bare feet, without shoes," she told the AFP news agency.
The city of Osh, which saw most of the violence, is being patrolled by Kyrgyz troops, amid an uneasy calm. However, there have been reports of soldiers taking part in looting.
The ICRC says its workers have reached refugees in the areas around Osh.
"We've seen for ourselves and also heard about pockets of displaced people ranging from several hundred to several thousand in number," said the ICRC's Severine Chappaz.
The organisation said insecurity and fear, combined with shortages of basic necessities like food, water, shelter and medicine, were putting a tremendous strain on communities, hospitals and families.
Many report instances of rape, and severe beatings.
"We need clothes and medical supplies, especially for the children, because when we fled our homes we just ran away and couldn't take anything with us," said Halima Otajonova, a 41-year-old mother of two, at a refugee centre at a stadium in the Uzbek town of Khanabad.
"Some of us even ran away in bare feet, without shoes," she told the AFP news agency.
The city of Osh, which saw most of the violence, is being patrolled by Kyrgyz troops, amid an uneasy calm. However, there have been reports of soldiers taking part in looting.
The ICRC says its workers have reached refugees in the areas around Osh.
"We've seen for ourselves and also heard about pockets of displaced people ranging from several hundred to several thousand in number," said the ICRC's Severine Chappaz.
The organisation said insecurity and fear, combined with shortages of basic necessities like food, water, shelter and medicine, were putting a tremendous strain on communities, hospitals and families.
Paul Quinn-Judge, Central Asia Project Director at the International Crisis Group, a Brussels-based security think tank, said the situation was likely to get worse.
"We're going to have an increasingly serious humanitarian problem which is going to affect both the Kyrgyz and the Uzbek communities in southern Kyrgyzstan," he told the BBC from the capital, Bishkek.
"The reports from the Uzbek communities in Osh and in Jalalabad are so bloodcurdling that I doubt whether anybody will want to go back in the near future. In fact they'd probably only go back if the Uzbek government forced them to."
In an Uzbek district of Osh, a baker who had fled to the border with his wife and five children on Sunday said his family had lost hope after supplies on the border ran out, and returned out of desperation.
"Is there any difference where to die? There is no food, no water, no humanitarian aid," Melis Kamilov told the Associated Press news agency. Mob attacks
Eyewitnesses say Kyrgyz mobs began attacking people in Uzbek areas of Osh and another southern city, Jalalabad, in the early hours of Friday last week.
Kyrgyzstan's interim leaders have been struggling to impose their authority since coming to power after President Kurmanbek Bakiyev was overthrown in April.
The government believes allies of Mr Bakiyev, who now lives in exile in Belarus, want to derail a national referendum on constitutional reform scheduled for 27 June.
But the government has said it will go ahead with the referendum despite the clashes.
Mr Quinn-Judge told the BBC the decision not to delay the vote suggested the interim government was "having a few reality problems".
Ethnic Uzbeks have largely supported the interim government, but Mr Bakiyev remains popular with many Kyrgyz in the south.
A Kyrgyz government appeal for Russia to send in peacekeeping troops was rejected by Moscow.
Russian Foreign Minister Sergei Lavrov has now said his country will provide technical assistance to Kyrgyzstan to help it track down those behind the clashes.
Washington is sending its top Central Asia diplomat, Robert Blake, for meetings with officials in Bishkek on Friday and Saturday.
The clashes are the worst ethnic violence to hit southern Kyrgyzstan since 1990, when deadly clashes were suppressed by the Soviet authorities.
"We're going to have an increasingly serious humanitarian problem which is going to affect both the Kyrgyz and the Uzbek communities in southern Kyrgyzstan," he told the BBC from the capital, Bishkek.
"The reports from the Uzbek communities in Osh and in Jalalabad are so bloodcurdling that I doubt whether anybody will want to go back in the near future. In fact they'd probably only go back if the Uzbek government forced them to."
In an Uzbek district of Osh, a baker who had fled to the border with his wife and five children on Sunday said his family had lost hope after supplies on the border ran out, and returned out of desperation.
"Is there any difference where to die? There is no food, no water, no humanitarian aid," Melis Kamilov told the Associated Press news agency. Mob attacks
Eyewitnesses say Kyrgyz mobs began attacking people in Uzbek areas of Osh and another southern city, Jalalabad, in the early hours of Friday last week.
Kyrgyzstan's interim leaders have been struggling to impose their authority since coming to power after President Kurmanbek Bakiyev was overthrown in April.
The government believes allies of Mr Bakiyev, who now lives in exile in Belarus, want to derail a national referendum on constitutional reform scheduled for 27 June.
But the government has said it will go ahead with the referendum despite the clashes.
Mr Quinn-Judge told the BBC the decision not to delay the vote suggested the interim government was "having a few reality problems".
Ethnic Uzbeks have largely supported the interim government, but Mr Bakiyev remains popular with many Kyrgyz in the south.
A Kyrgyz government appeal for Russia to send in peacekeeping troops was rejected by Moscow.
Russian Foreign Minister Sergei Lavrov has now said his country will provide technical assistance to Kyrgyzstan to help it track down those behind the clashes.
Washington is sending its top Central Asia diplomat, Robert Blake, for meetings with officials in Bishkek on Friday and Saturday.
The clashes are the worst ethnic violence to hit southern Kyrgyzstan since 1990, when deadly clashes were suppressed by the Soviet authorities.
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