The clashes have killed up to 2,000 people and set off a massive wave of refugees, with 400,000 people crammed in squalid camps with little access to clean water and food on Kyrgyzstan's sun-parched border with Uzbekistan.

After talks with Kyrgyz officials, Assistant Secretary of State Robert Blake said an international investigation must be held into the possible causes of the violence.

Kyrgyzstan's tiny, under-equipped army has struggled to bring order to the south and relief organizations have been unable to reach the worst-affected areas for security reasons.

Some Kyrgyz officials have said a referendum on a new constitution, due be held on June 27, should be postponed until the situation stabilizes.

The United States and Russia, both operating military air bases in the strategic Muslim nation, are concerned that continued turmoil in Kyrgyzstan could spread to other parts of Central Asia, a vast former Soviet region north of Afghanistan.