TASHKENT, Uzbekistan (AP) – Uzbek police are rounding up activists in a new crackdown, opposition leaders said Monday, as Sen. John McCain repeated demands for an inquiry into this month’s violent uprising, calling it a “massacre.”

The protest in the eastern city of Andijan exploded into violence when militants seized a local prison and government headquarters and thousands of people demonstrated in the streets.

Uzbek authorities say 173 people died in the May 13 violence, and deny they opened fire on unarmed civilians. Human rights advocates say up to 750 people were killed.

President Islam Karimov – viewed as one of the most authoritarian leaders still in control of a former Soviet republic – has rejected U.N. and Western calls for an international inquiry, saying Uzbek authorities would conduct their own probe.

“Karimov must understand that this kind of activity has no place in the 21st century,” McCain said in the Kyrgyz capital Bishkek after he and tw other U.S. senators met with U.S.-funded humanitarian organizations. “We repeat our demand for a full and complete investigation … of the massacre that occurred just a few days ago.”

Karimov blamed the unrest on Islamic extremists, accusing them of killing hostages and of using civilians as human shields.

Uzbekistan became a key U.S. ally after the Sept. 11, 2001, attacks, offering use of Khanabad air base near the Afghan border – instrumental for U.S. forces who ousted the hardline Taliban from power in Afghanistan.

The United States and Uzbekistan signed a strategic partnership agreement in 2002 and Defense Secretary Donald H. Rumsfeld has praised Uzbekistan’s support in the war on terror.

But McCain, R-Ariz., warned that the Uzbek government’s failure to allow an international probe of the events in Andijan, Uzbekistan’s fourth-largest city, would jeopardize U.S.-Uzbek ties.

“I think the United States’ relationship is directly related to the actions taken by President Karimov,” he said, sitting next to two other Republican senators, Lindsey Graham of South Carolina and John Sununu of New Hampshire.