Kyrgyzstan's interim government has asked Russia to help end ethnic clashes in the southern city of Osh, in which 77 people have been killed.
"We need the entry of outside armed forces to calm the situation down," interim leader Roza Otunbayeva said.
But Russia said for the time being it would not send troops to Kyrgyzstan.
Thousands of ethnic Uzbeks have massed at the Kyrgyz-Uzbek border as they try to escape fighting between Kyrgyz and Uzbeks that has raged for two days.
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".
Russia was not prepared to send troops to Kyrgyzstan under the current circumstances, but would send humanitarian aid to the violence-hit region, a spokeswoman for Russian President Dmitry Medvedev said.
"It is an internal conflict and for now Russia does not see the conditions for taking part in its resolution," Natalya Timakova, Mr Medvedev's spokeswoman, was quoted as saying by Russian news agencies.
Mr Medvedev would consult with other members of the regional security grouping, the Shanghai Co-operation Organisation, about a response to the crisis, Ms Timakova said.
The 150 Russian soldiers based near the Kyrgyz capital had been given no instruction to intervene, Russian media quoted an unnamed senior military source as saying.
A plane carrying medical supplies has been sent to Kyrgyzstan to evacuate some of the badly-injured people for treatment in Russia, officials in Moscow said.
The violence is the worst to hit the Central Asian country since President Kurmanbek Bakiyev was overthrown in April.
Nearly people have been hurt, health ministry officials said, adding that there was a shortage of food and care for the injured.
The BBC's Rayhan Demytrie, who is at the Kyrgyz-Uzbek border, says guards have been letting people through one by one.
The refugees 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.
A state of emergency was declared in a second southern city, Jalalabad, as "the area of ethnic instability is expanding", an official with the interim government said.
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.
Ms Otunbayeva has blamed supporters of Mr Bakiyev for inciting the violence to destabilise the country ahead of the referendum.
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.