The White House voiced skepticism Monday about a tentative agreement reached over the weekend in which Russia would enrich uranium for Iran.

On Sunday, Russia and Iran agreed in principle to joint uranium enrichment venture on Russian territory, a development that could be a breakthrough in the international showdown with Tehran over its suspected nuclear ambitions. The Bush administration has supported the Russian proposal, as long as all enrichment activities take place outside of Iran and all spent fuel is returned to Russia.

But — noting that further negotiations on the details of the agreement lay ahead and that an Iranian official has been quoted saying Tehran intends to continue uranium enrichment on its own soil as well — the White House voiced doubts that the deal meets the concerns of the United States and other countries.

"We'll have to see what the details of any agreement are," White House spokesman Scott McClellan said. "Given their history, you can understand why we remain skeptical."

Russia said talks with Iran had not ended and would continue until the International Atomic Energy Agency board of governors' meeting on Iran next week. That meeting could start a process leading to punishment by the U.N. Security Council, which has the authority to impose sanctions on Iran.