Uzbek troops opened fire repeatedly on demonstrators during unrest May 13 in the city of Andijon, in one incident mowing down most of a crowd of 300 that was trying to break free from troop encirclement, Human Rights Watch said Tuesday.

In a report drawing on interviews with 50 witnesses in and outside the country, the research and advocacy group said some of the men at the protest took hostages and used them as human shields. "But neither these crimes nor the peaceful protests that ensued can justify the government's response," the report said, terming the actions a "massacre."

The New York-based group called on the United States to suspend negotiations with Uzbekistan over long-term use of a military base until the government there agrees to an international, independent investigation into the violence.

Kenneth Roth, executive director of Human Rights Watch, told reporters in Moscow that a permanent American base and the accompanying injection of cash and investment that the government would receive "would be utterly inappropriate for a government that has just slaughtered hundreds of its citizens."

The U.S. military is already using a base in southeastern Uzbekistan on a temporary basis to support operations in Afghanistan. The United States has also provided military aid to Uzbekistan under a counter-terrorism program, but it is unclear whether units that have received U.S. aid took part in the Andijon killings. U.S. law requires the suspension of aid if recipient foreign forces commit gross human rights violations.

State Department spokesman Sean McCormack declined to comment on the report, saying he had not seen it. He said that "we continue to urge Uzbekistan . . . to undertake a credible and transparent assessment of the tragic events in Andijon, in cooperation with an international partner, as well as undertake fundamental, democratic and economic reforms."

The International Committee of the Red Cross said Tuesday that the government had refused it access to Andijon, where it wants to check the condition of injured and arrested people.

The Human Rights Watch report drew on interviews with witnesses who fled to neighboring Kyrgyzstan after the violence and residents of Andijon who spoke to one of the group's researchers who worked clandestinely in the city for three days. The report offered no exact death toll but said "hundreds of unarmed people" were killed.

The Uzbek government has said that 173 people died, most of them Islamic radicals. An unidentified Uzbek official in Tashkent, the capital, told the Reuters news agency Tuesday that a government response to the report was not ready.

The 63-page document's detailed chronology of events begins around midnight on May 12-13 when 50 to 100 men attacked a police station and a military barracks. The men included relatives and supporters of 23 local businessmen who were on trial on charges of "religious extremism," allegations that they and their supporters deny. The men seized "a significant number of weapons, including automatic AK-47 rifles and grenades, as well as a Zil-130 military truck," the report says.

The Uzbek government later said the two attacks killed four policemen and two soldiers.

"The attackers, who referred to each other as 'brothers' and may have been members of an informal 'brotherhood' of devout Muslims, would remain a cohesive group throughout the unfolding events," the report says.

The men then stormed the local prison, freeing the 23 defendants and hundreds of others while killing three prison guards. Skirmishes continued through the night, including a heavy gun battle outside the National Security Service building, in which as many as 15 attackers may have been killed. The gunmen also seized a number of hostages, including policemen, firemen, the local tax inspector, a judge and the city prosecutor.

By late morning on May 13, "the protest was transformed from the actions of several dozen armed gunmen into a massive expression of dissatisfaction with the endemic poverty, corruption, unemployment, repression and unfair trials that plagued the area," the report says. Security forces shot into the edges of the crowd several times.

Up to 10,000 people were in the square in the afternoon. Security forces began to surround it about 4 p.m. and began a full-scale assault about 5:20, scattering the crowd with gunfire. One group of 300 people who fled the square to an adjoining street with a number of hostages found their exit blocked and were mowed down by soldiers firing from armored vehicles, from behind sandbags and from rooftops, the report says. It says that there were few survivors and that hostages were also killed by security forces.

A woman grieves near small, numbered plaques said to mark the graves of crackdown victims in a cemetery on the outskirts of the city of Andijon.