At least 176 people have been killed around the southern cities of Osh and Jalalabad in violence that began on Thursday and escalated into the deadliest clashes the impoverished Central Asian state has seen in 20 years.

As the violence escalated over the weekend, witnesses said gangs armed with automatic rifles, iron bars and machetes set fire to houses and shot fleeing residents.

A Reuters reporter said Osh appeared considerably calmer on Tuesday but the atmosphere remained tense. A senior US official told Reuters in Vienna that the violence appeared to have stopped spreading and might be receding.

The events have fueled concern in Russia and the United States, both of whom operate military air bases in the strategic but volatile nation west of China.

UN officials have said the number of ethnic Uzbeks fleeing the clashes for neighboring Uzbekistan may soon exceed 100,000 and warned against a spreading of the violence outside Kyrgyzstan's borders.

"It seems indiscriminate killings, including of children, and rapes have been taking place on the basis of ethnicity," said Navi Pillay, United Nations High Commissioner for Human Rights.

She urged Kyrgyz authorities to take "swift and decisive action" to protect people irrespective of their ethnic origin.

Interim leader Roza Otunbayeva, who took power after a deadly revolt in April that ousted president Kurmanbek Bakiyev, was quoted by Russia's Interfax news agency as saying the real death toll from the violence could be "several times" higher than the official numbers.

The interim government has said more violence could occur in the capital Bishkek and another region of the north.

"We are keeping enough forces in Bishkek and (we) are working to ensure that Bishkek stays under our control," said Otunbayeva who has accused supporters of Bakiyev, who is now in exile in Belarus, of stoking ethnic conflict.

Bakiyev has denied that he is behind the violence. His son Maxim was arrested in Britain after he landed at an airport in southern England, Kyrgyz mass media have reported.

Pillay's office said on Tuesday that the violence appeared to have begun with five coordinated attacks by men carrying guns and wearing balaclavas.

"We have strong indications that this event was not a spontaneous inter-ethnic clash — that it was to some degree orchestrated, targeted and well-planned," her spokesman Rupert Colville told a briefing in Geneva.

Analysts say that if southern Kyrgyzstan, which shares the densely populated Ferghana Valley with Uzbekistan and Tajikistan, descends into chaos, it could help militants gain a firmer footing in the volatile region.

United Nations Special Envoy Miroslav Jenca said Kyrgyzstan should take every step possible to ensure that violence did not spread to other parts of ex-Soviet Central Asia, a vast Muslim region north of Afghanistan and Iran.

"The most important task now is to stop the bloodshed," Jenca told reporters. "This conflict should be localized."

Interim leader Otunbayeva said Uzbekistan had shut the border to block "those seeking revenge."

"Otherwise it would have already been a war between the two states," she said.

Nancy McEldowney, US principal deputy assistant secretary for European and Eurasian affairs, said the violence appeared to be easing.

"I know that there was a concern initially about the violence spreading and we saw the violence spread from Osh to Jalalabad," she said.

"But reports over the course of the last 24 hours have been that that spreading had abated," she said on the sidelines of an Organization for Security and Cooperation in Europe (OSCE) meeting in Vienna.

The region's main security bloc, the Russian-led Collective Security Treaty Organization (CSTO), on Monday proposed sending helicopters and equipment to help stop the violence.

But Kyrgyzstan said it had been told not to expect the immediate dispatch of troops.

The CSTO is seen as the most likely vehicle for a deployment of peacekeepers from Russia, which is vying for influence with China and the United States in the region.

"Moscow greatly fears instability in this region," Eurasia Group analysts said in a note. "The violence poses the prospect of a lawless area in the south of Kyrgyzstan that could, in the Kremlin's view, eventually provide safe harbor to Islamic militants and ease the operating environment for organized crime and narco-trafficking groups."

Washington uses its air base at Manas in the north of the ex-Soviet republic, about 300 km from Osh, to supply forces fighting the Taleban in Afghanistan.

The United Nations and the European Union have urged the interim government to stick to plans for a referendum on June 27 and parliamentary elections in October. Otunbayeva said on Tuesday there were no plans to delay the referendum.

The violence is the worst in Kyrgyzstan since 1990, when Soviet leader Mikhail Gorbachev sent troops to Osh after hundreds were killed in a row that started over land ownership.