Days of communal violence in Kyrgyzstan have sparked an "immense crisis" in the area, the Red Cross (ICRC) says.
Some aid has begun to arrive in the region, but the ICRC says refugees are running short of basic supplies such as food, water and shelter.
Tens of thousands of people have fled their homes because of the violence between ethnic Kyrgyz and Uzbeks.
Troops are now patrolling the riot-hit streets of the southern city of Osh, amid an uneasy calm.
At least 191 people have been killed in the clashes, the health ministry said, although some observers have said the death toll is higher.
According to ICRC workers, "several hundred people" have been killed in the fighting.
The ICRC said the lull in fighting had allowed its workers to reach 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.
It had also heard accounts of rape and beatings. And there have been reports of soldiers looting food aid in Osh.
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.
Some 75,000 ethnic Uzbeks are thought to have crossed the border into Uzbekistan, and as many as 200,000 more have fled their homes.
Many are being housed in temporary shelters, but the fate of many others is still unknown.
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.