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.
Andijan remains sealed off from the rest of the country
Secretary of State Condoleezza Rice called for political reform in the country following the recent violence.
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, which is a key US ally in the region.
UK Foreign Minister Jack Straw is expected to raise the reported killings when he visits Washington on Tuesday.
Mr 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.
Human rights organisations and a leading US Republican congressman have called on the Bush administration to condemn President Islam Karimov.
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
He has blamed the unrest on Islamic extremists.
Ms 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," she said.
"This is a country that needs... pressure valves that come from a more open political system."
Earlier, the US called on the Uzbek government to allow the International Red Cross full access to the part of the country affected by recent protests.
However, US state department spokesman Richard Boucher also condemned violent protesters who had stormed government buildings.
The state department had previously called for all sides to exercise restraint.
The US has a base in Uzbekistan, which is used by coalition troops engaged in Afghanistan.
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.
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, which is currently said to be calm.
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.