Kyrgyzstan's interim government has asked Russia to help quell ethnic fighting in the southern city of Osh, in which 50 people have been killed.
"We need the entry of outside armed forces to calm the situation down," interim leader Roza Otunbayeva said.
Buildings are ablaze and gunfire can be heard as fighting between Kyrgyz and ethnic Uzbeks rages for a second day.
Thousands of ethnic Uzbeks are trying to cross into Uzbekistan. One child was crushed to death at the border.
In a televised address, Ms Otunbayeva said she had sent a letter to Russian President Dmitry Medvedev asking him to send military reinforcements.
She described the situation in Osh as "out of control".
More than 650 people have been hurt, health ministry officials said, adding that there was a shortage of food and care for the injured.
It is the worst violence the Central Asian country has seen since President Kurmanbek Bakiyev was overthrown in April.
'Streets ablaze'
The BBC's Rayhan Demytrie, who is at the Kyrgyz-Uzbek border, says guards have been letting people through one by one.
She says everybody is distressed, women are in tears and are asking for international help. They say their houses were burned down and they have nowhere to go.
People at the border say that hundreds of other Uzbeks are stranded in their neighbourhoods in Osh.
They claim that people are being shot at from armoured personnel carriers, which are making way for armed gangs of ethnic Kyrgyz.
Eyewitnesses told the BBC that many people had been shot dead but it was impossible to retrieve the bodies.
"The situation is very bad. There is no sign of it stopping. Homes have been set ablaze... Entire streets are on fire," said Interior Ministry spokesman Rakhmatillo Akhmedov.
Machine-gun fire can be heard as troops try to regain control of the streets, the Associated Press reports.
The interim government has denied security personnel are shooting at people and says its forces are preventing others from entering Osh to join the fighting.
Ethnic tensions
Kyrgyz TV reported that the country's borders with Tajikistan and China have been closed because of the unrest.
According to local reports, the violence broke out between rival gangs and developed into gun battles late on Thursday.
Riots and looting followed.
The unrest comes ahead of a referendum on the constitution, scheduled for 27 June.
The leaders of Russia and China - which borders Kyrgyzstan - have appealed for calm.
The political crisis has raised fears of a civil war in the country, where both Russia and the US have military bases.
Washington uses the base about 300km (190 miles) from Osh for US operations in Afghanistan.
Mr Bakiyev fled with his family to Belarus after violent clashes between government forces and protesters on 7 April.
