Kyrgyzstan's interim government has given security forces shoot-to-kill powers in a bid to stop ethnic fighting which has taken nearly 80 lives.
It also declared a partial mobilisation of the army to combat "destructive forces and criminal elements".
Thousands of ethnic Uzbeks have been fleeing what they say are ethnic Kyrgyz gangs in the southern city of Osh.
Almost 1,000 people were also hurt in the worst unrest since President Kurmanbek Bakiyev's overthrow in April.
Russia says it does not plan to intervene despite a Kyrgyz request.
And without international assistance there are fears the interim authorities in Kyrgyzstan may struggle to contain the conflict, the BBC's Rayhan Demytrie reports.
The south of Kyrgyzstan, an ex-Soviet Central Asian state of 5.5 million people, is home to an ethnic Uzbek minority of almost one million.
The latest violence has become the biggest challenge for the new government so far.
'Worse by the hour'
A decree from the interim government authorised security forces to shoot to kill in the Osh and Jalalabad regions.
Lethal force was permitted in areas placed under a state of emergency in order to defend civilians, in self-defence, and in the event of mass or armed attacks, the decree said.
Uzbek eyewitnesses told our correspondent at a border crossing with Uzbekistan that gangs of armed Kyrgyz had been marauding through neighbourhoods, killing residents and burning homes.
One woman pleaded for help: "We need food, we need water, I have got two sons and they are little and I need water and food to survive."
The International Committee of the Red Cross said it had received reports of tens of thousands people fleeing fighting and looting.
"Things are getting worse and worse by the hour," said Severine Chappaz, deputy head of the ICRC's mission in Kyrgyzstan, in a statement sent from Osh to the Associated Press news agency.
"The electricity and gas have been cut off, meaning there are also no water supplies. Shops and markets are closed, leading to fears of a lack of food, especially in the hospitals and places of detention."
There have also been reports of Kyrgyz casualties and violent clashes between the two ethnic groups have been continuing for two days.
'Out of control'
The interim Kyrgyz leader, Roza Otunbayeva, went on national TV to say she had sent a letter to Russian President Dmitry Medvedev asking him to send military help.
"The situation in the Osh region has spun out of control," she told reporters.
"Attempts to establish a dialogue have failed, and fighting and rampages are continuing. We need outside forces to quell confrontation."
A spokeswoman for the Russian president said Moscow was not prepared to send troops under the current circumstances but would send humanitarian aid.
"It is an internal conflict and for now Russia does not see the conditions for taking part in its resolution," Natalya Timakova said.
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 added.
Russia has close ties to its fellow ex-Soviet republic and keeps several hundred service personnel at an air base it maintains there.
Mr Bakiyev fled with his family to Belarus after being ousted.