"I cannot name a date" as to when the crucial agreement will be signed, Alexander Rumyantsev told AFP. "We have not agreed with Iran about anything yet."
Russia is building the Islamic state's first nuclear power plant at Bushehr, but has said it would not begin delivering the fuel for the plant until Tehran signs a deal to return the spent fuel to Russia.
On Tuesday, attending a general conference of the International Atomic Energy Agency (IAEA) in Vienna, Rumyantsev said Russia was close to resolving the spent fuel dispute.
Under pressure from the United States and Israel -- which fear that Iran is developing nuclear weapons through the Bushehr project -- Russia has made this a key condition for concluding its 800 million dollar (715 million euro) deal.
"There is nothing preventing us from agreeing.... We will agree," Rumyantsev said Tuesday.
However speaking on the sidelines of a meeting in Moscow with US Energy Secretary Spencer Abraham, Rumyantsev said the negotiating process with Iran was "taking a long time -- these are prolonged discussions."
But he also tried to play down the confusion over the deal, which has complicated Moscow's relations with Washington on the eve of a key summit later this month between Russian President Vladimir Putin and his US counterpart George W. Bush at Camp David.
"I don't see any sensation in this," he said referring to Russia's failure to sign the agreement.
"This is a normal process," he said.
Russian official have said that negotiations over Bushehr have broken down over Iran's demand for Russia to buy back the spent fuel.
The demand is highly unusual since spent fuel in such deals is almost always sent back for free, and the Iranian demand was not part of the original Bushehr contract.
Russia has further called on Iran to abide by the IAEA's deadline for Iran to prove by October 31 that it is not secretly developing atomic weapons.
Iran's first nuclear power station in Bushehr is due to go online in 2005.
Washington, which suspects Tehran of pursuing a nuclear weapons program, has been pressing Russia strongly not to sign an accord.
Abraham is in Moscow to sign the Nuclear Cities Initiative, a program aimed at enhancing US and global security by supporting weapons reduction in Russian cities where nuclear weapons are stationed or nuclear sites are located.