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 blazed and gunfire could be heard as fighting between Kyrgyz and ethnic Uzbeks raged this weekend, after violence broke out earlier in the week.

Thousands of ethnic Uzbeks tried to cross into Uzbekistan. One child was crushed to death at the border.

More than 650 people have been hurt in the violence that began on Thursday, health ministry officials said.

A state of emergency and a curfew has been imposed.

It is the worst violence the Central Asian country has seen since President Kurmanbek Bakiyev was overthrown in April.

Troops have been deployed on the streets of Osh. "Entire streets are on fire," said interior ministry spokesman Rakhmatillo Akhmedov.

"The situation is very bad. There is no sign of it stopping. Homes have been set ablaze," he said.

Most of the properties attacked appeared to belong to ethnic Uzbeks, reports say.

Police and soldiers struggled to stop the violence as gangs of armed young Kyrgyz men marched on Uzbek neighbourhoods where houses were already burning.

The real casualty figures may be higher than reported because doctors and human rights workers said ethnic Uzbeks were afraid to seek hospital treatment.

The situation poses a decisive test of the provisional government's ability to control the country, where the US and Russia both have military air bases.

The government needs stability to hold a 27 June vote on a new constitution and go ahead with elections for a new parliament in October.

"Crowds of 100 to 150 people are continually passing my house," said Bakyt Omorkulov, a member of the Coalition for Democracy and Civil Society, a non-governmental organisation.