Scott McClellan, White House spokesman, told reporters on Monday: "We'll have to see what the details of any agreement are.
"Given their history, you can understand why we remain sceptical."
On Sunday, Russia and Iran agreed in principle to a joint uranium enrichment venture on Russian territory, a development that could be a breakthrough in the international showdown over Iran's suspected nuclear ambitions.
The Bush administration has supported the Russian proposal, as long as all enrichment activities take place outside Iran and all spent fuel is returned to Russia.
But in his comments McClellan noted 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.
"Given their history, you can understand why we remain sceptical"
White House spokesman
As such, he said, the White House doubted that the deal met the concerns of the United States and other countries.
Russia said talks with Iran had not ended and would continue until the International Atomic Energy Agency (IAEA) board of governors' meeting on Iran next week.
That meeting could start a process leading to punishment by the UN Security Council, which has the authority to impose sanctions on Iran.
Facing that threat, Mahmoud Ahmadinejad, Iran's president, spent Monday seeking to reassure regional states about Tehran's nuclear programme, saying his country was a "good neighbour" that wanted regional stability.
Ahmadinejad is keen to reassure
his Gulf neighbours
"We want peace, security, progress for all the countries of the region, especially our neighbours," he told reporters during a brief visit to Kuwait.
"History has shown that Iran is a good neighbour for the countries in the region. We are just working on nuclear fuel for peaceful purposes," he added, speaking through a translator.
Meanwhile, Hosni Mubarak, the Egyptian president, also touring Gulf Arab states, said in remarks aired by Aljazeera that his country wanted a peaceful solution to the stand-off between Tehran and the West over its nuclear plans.
"They must reach a solution because a confrontation is very dangerous for the region and the Iranians ... we do not want a confrontation and we do not want the use of force against Iran," he said in Qatar.
His Qatari host, Amir Shaikh Hamad bin Khalifa Al Thani, said Tehran appeared to be pushing ahead with its programme.
"We hope they reach a solution, but it is almost clear that the Iranians are pressing ahead with their projects," he said.