US officials in Cambodia welcomed the arrest of Ieng Sary and Ieng Thirith Monday, calling it a move in the right direction for the tribunal.
The arrests will not affect the so-far absent US funding for the tribunal.
The tribunal must still prove itself up to international standards before the US would support it through funding, US Embassy spokesman Jeff Daigle said.
"We are evaluating that right now," he said.
The $56 million tribunal is supported mostly by the UN. But the government and other observers say the special courts will run out of money before they are able to go through full proceedings.
Foreign Minister Hor Namhong said last month proceedings could last through 2010, well beyond the current tribunal budget.