The US says it is "deeply disturbed" by reports that troops in Uzbekistan fired on unarmed civilians during a protest in the east of the country.
Uzbek authorities have sealed off the town of Andijan
It called on the Uzbek government to allow the International Red Cross full access to the part of the country affected by recent protests.
Local sources say several hundred people died when troops shot at unarmed protesters in Andijan on Friday.
The US has been under pressure to take a tough line against the Uzbek regime.
President Islam Karimov has blamed the unrest on Islamic extremists.
His government is a key US ally in the region - the US has a base in Uzbekistan, which is used by coalition troops engaged in Afghanistan.
US state department spokesman Richard Boucher said that stability in Uzbekistan depended on the government addressing human rights issues and the rule of law.
Most populous central Asian former Soviet republic, home to 26m people
Ruled since 1991 independence by autocrat Islam Karimov
Accused by human rights groups of serious abuses, including torture
Rocked by violence in capital Tashkent in 2004
Government says radical Islamic groups behind violence
However, he also condemned violent protesters who had stormed government buildings.
Human rights organisations and a leading US Republican Congressman, Dana Rohrabacher, have called on the Bush administration to condemn Mr Karimov.
The state department had previously called for all sides to exercise restraint.
Secretary of State Condoleezza Rice called on Mr Karimov, who has been in power since the collapse of the Soviet Union, to relax his grip on power.
"We have been encouraging the Karimov government to make political reforms, to make the system more open," she said.
UK Foreign Minister Jack Straw, who has condemned the violence, said his Uzbek counterpart had pledged to allow diplomats access to Andijan on Tuesday.
However, the Uzbek foreign ministry made no mention of any such agreement.
The city is currently sealed off. Journalists forced to leave say they saw more than 30 police checkpoints on the main road to the capital, Tashkent.
The protests were sparked by a long-running trial of local businessmen accused of Islamic extremism. Their families say they are innocent and have been unfairly targeted.
Witnesses say government troops attacked mostly unarmed demonstrators in the town, killing possibly hundreds of them.
A Ukrainian television crew that reached Andijan soon after the shooting reported seeing many bodies in the streets, and said many children had been killed.
The Uzbek authorities say soldiers did not shoot at anyone, apart from gunmen from a radical Islamist group.
The unrest spread to the eastern border town of Korasuv, where locals seized control of government buildings on Saturday. Uzbek troops have now sealed off the town.
The town is currently calm, although there are rumours that the central authorities may move to take control, says the BBC's Ian MacWilliam in Kyrgyzstan.
The authorities in Kyrgyzstan have registered more than 500 refugees from Andijan.
Some said troops shot at them as they tried to cross the border and some died.
The refugees, mostly men but including some women and small children, say they fear government reprisals if they return to Uzbekistan.