"We repeat our demand for a full and complete investigation ... of the massacre that occurred just a few days ago," Senator John McCain, a Republican from Arizona, said in the Kyrgyz capital Bishkek after a meeting with US-funded humanitarian organisations on Monday.
"[Uzbek] President [Islam] Karimov must understand that this kind of activity has no place in the 21st century," McCain said, adding that the inquiry should be conducted by the Organisation for Security and Cooperation in Europe.
He warned that the Uzbek government's failure to allow an international investigation of the events in Andijan, Uzbekistan's fourth-largest city, would jeopardise US-Uzbek ties.
Kyrgyz to allow over 500 Uzbek
refugees to remain in country
"If these kinds of actions continue on the part of President Karimov, including a failure to allow a complete and thorough investigation by an outside organisation such as the OSCE, then the United States will have to react accordingly." McCain said, sitting next to two other Republican senators, Lindsey Graham of South Carolina and John Sununu of New Hampshire.
McCain said that the Kyrgyzstan government would allow more than 500 refugees who had fled the violence in Uzbekistan to remain until "conditions are such that they can return without fear".
Also on Monday, an opposition party leader said Uzbek police had detained dozens of opposition activists over the weekend in a new crackdown on dissent after this month's uprising in eastern Uzbekistan.
"Within the last two days, police have detained dozens of our party members, saying we are hiding terrorists involved in the recent uprising in the Fergana Valley," Vasilya Inoyatova, the leader of the outlawed Birlik, or Unity, movement said by phone from a police station in the capital Tashkent.
A human rights activist, Surat Ikramov, on Monday said that police were preventing him from leaving his home in Tashkent and that he had received calls from numerous other rights activists who either had been detained or were forcibly isolated in their homes.
"Within the last two days, police have detained dozens of our party members..."
Birlik movement leader
The detentions follow the uprising that erupted in the eastern city of Andijan on 13 May, when rebels seized a local prison and government headquarters and thousands of people demonstrated in the streets.
Uzbek authorities say 173 people died, but deny they opened fire on unarmed civilians. Rights advocates say up to 750 people were killed in the violence.