"We've now seen about 100,000 military-aged men missing ... They may have been murdered," Mr Cohen said on CBS television.
Refugees leaving Kosovo have told relief workers of Serb troops separating men from women and children and not allowing them to cross the border, presumably to try to stop them from joining the Kosovo Liberation Army.
Mr Cohen said he had already received reports that up to 4,600 Kosovo men have been executed. "But I suspect it's far higher than that."
Serbian forces have so far displaced 1.5 million ethnic Albanian men, women and children in Kosovo, he said.
Aid workers in Macedonia said about 600 Kosovo refugees had entered the country yesterday and 300 on Saturday, as numbers continued to rise after a 10-day pause. The aid workers said their arrival might herald the start of a new exodus, similar to the days in April and early May.Reuse content