The United States tonight finally broke its silence on the reported massacre of hundreds of unarmed citizens gunned down by security forces in Uzbekistan.

The State Department said it was "deeply disturbed" by the reports. We certainly condemn the indiscriminate use of force against unarmed civilians and deeply regret any loss of life," said spokesman Richard Boucher.

But American officials tip-toed around direct criticism of the regime of President Islam Karimov.

Condoleezza Rice, the US Secretary of State, called on Mr Karimov to adopt political reforms to head off future outbreaks of violence. "This is a country that needs, in a sense, pressure valves that come from a more open political system," she said of the Central Asian republic that is a key American ally in the War on Terror.

Ms Rice, flying back to Washington after her surprise visit to Iraq, herself offered no censure of Uzbekistan despite the reports, several days old, of a massacre in the city of Andijan last week. She said Washington’s main concern was to prevent further violence, but made no public appeal to Mr Karimov.

Her tempered response put America at odds with Britain, where Jack Straw, the Foreign Secretary, issued a firm denunciation of the violence. "This plainly cannot be justified," he said, adding that he was "very concerned".

Mr Straw refused to accept Mr Karimov’s explanation for the violence at face value. The Uzbek leader, who has ruled with an iron fist since the republic won its freedom from the former Soviet Union 14 years ago, blamed it on Islamic extremists. It was impossible to verify "in any precise way" Mr Karimov's version of events, said Mr Straw, because the Uzbek authorities have refused to allow diplomats, aid workers or reporters to visit Andijan, access that Mr Straw demanded immediately.

The violence, which is threatening to spread across eastern Uzbekistan amid reports of more protests and fighting in towns and villages near Andijan, places the White House in an awkward dilemma. Mr Bush has made the "end of tyranny" the theme of his second term. But Mr Karimov, whose style of government would qualify under most definitions of tyrannical rule, has been especially helpful to the American administration. He has given the Pentagon free run of a military base close to the Afghan border that acted as the hub for special forces operations in the 2001 war against the Taleban and has since given the United States a priceless "footprint" in Central Asia.

In Uzbekistan, tensions were mounting on the border with Kyrgyzstan as hundreds of Uzbeks, including some rebels, tried to cross the closed frontier to escape Uzbekistan's worst violence since independence in 1991. Uzbek troops opened fire on anti-government demonstrators in Andijan on Friday, killing hundreds of people, according to eye witnesses, after armed rebels took over government buildings and freed prisoners from jail.

Saidjahon Zaynabitdinov, the head of the local human rights group, Appeal, said that troops had killed about 200 demonstrators in Pakhtabad on Saturday, although there was no independent confirmation.

Police in the town of Pakhtabad, about 18 miles northeast of Andijan, said today they had repelled a group of armed men who tried to force their way across the border to Kyrgyzstan. Kyrgyz authorities said they had also turned back about 150 Uzbek refugees trying to cross the border illegally near the Uzbek villager of Ayim and warned that they would quickly deport others.

At least eight government soldiers and three civilians were killed in clashes in the border town of Teshiktosh on Sunday as hundreds of Uzbeks fled into Kyrgyzstan, witnesses said. The UN refugee agency, UNHCR, said that 560 people, mainly men, had crossed the border into Kyrgyzstan as refugees since the unrest began and that Kyrgyz authorities were bracing for many more.

"There is a great deal of worry on the ground," a UNHCR spokesman, Peter Kessler, told BBC Radio. "Obviously the Kyrgyz authorities are preparing for the worst."

In another border community, Korasuv, an estimated 5,000 people went on a rampage on Saturday, beating up local officials and forcing them to restore a bridge across a river that marks the border with Kyrgyzstan.

"Every day, people are becoming more and more angry because of the lawlessness of the army. People think that those who left for Kyrgyzstan will return with arms. They are afraid," Muzafarmizo Iskhakov, the chairman of the human rights group, Ezgulik, told The Times from Andijan.

Andijan itself remained extremely tense with residents reporting bursts of gunfire and some saying troops were still fighting militants in Bogishonol, an outlying district of the city.

President Karimov has blamed the uprising on a branch of the banned Islamic group Hizb ut-Tahrir, which calls for setting up an Islamic state in Central Asia. His government denies that troops opened fire on the protester and says ten soldiers and about 70 insurgents were killed. But several witnesses say troops fired indiscriminately into the crowd, killing as many as 500 people.

Mr Straw said the British ambassador in Tashkent, David Moran, had met the Uzbek foreign minister to "call for immediate access" for UN and Red Cross officials, as well as ambassadors and journalists. The minister, Elyor Ganiyev, had agreed to organise a visit for diplomats and media today, he said.

"We have always accepted the need across the world for firm action to deal with terrorism, but that has been in the context in which there is respect for human rights, and in addition to that, where there is progress for democracy," Mr Straw said. Human rights groups say there are about 6,000 political and religious prisoners in Uzbekistan and some activists say as many as 4,000 may have been killed since 1991.

A group of opposition and human rights campaigners rallied in the Uzbek capital, Tashkent, today the first public show of anger there since the violence in Andijan. About 30 activists and independent Uzbek reporters laid flowers in central Tashkent in memory of those who were killed. "It was a black day in Uzbek history. We are ashamed," said Tashpulat Yuldashev, a political analyst. "We dissidents have been long afraid of standing up to express our discontent. But this time we can't stay silent."

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