"There are no differences on the essence of the legally binding document on nuclear weapons reductions," the Interfax-Military News Agency quoted Ivanov as saying during a trip to Greece. "There are certain differences concerning control mechanisms and approaches to the cuts."
Bush has promised to cut the U.S. arsenal to 1,700 to 2,200 strategic nuclear warheads, while President Vladimir Putin has said Russia could go as low as 1,500. Both countries are allowed 6,000 under the existing START I treaty.
The pledges last fall reflected the general improvement of U.S.-Russian ties, warmed by Putin's support for the U.S.-led war in Afghanistan. Bush initially favored an informal weapons deal but later agreed to Putin's push to formalize the cuts in a written agreement.
"Despite the increased level of trust between Moscow and Washington, mechanisms of control over nuclear weapons reduction must be preserved," Ivanov said. "All processes in this field must be controllable and predictable."
Talks held over the winter on the planned cuts were rocky because of Moscow's vociferous objection to U.S. plans to store decommissioned nuclear weapons for possible future use rather than destroying them.
Ivanov softened the Kremlin stance on a trip to Washington last month, saying that Russia wouldn't mind if the United States stores some of the weapons decommissioned under a new arms reduction pact.
He said Friday that Russia might follow the U.S. approach and put some weapons in reserve instead of dismantling all of them.
"There is some logic in this," Ivanov said. "I understand the Americans and do not rule out the possibility that this would serve our interests, too."