Russia's top nuclear official will sign a deal Saturday to supply Iran with fuel for its first nuclear reactor, an Iranian official said. The agreement has safeguards meant to banish fears of misuse for nuclear weapons (search) — but it is sure to add to U.S. concerns a day after the U.S.-Russian presidential summit.

The United States and Israel fear the Iranians could use the Bushehr reactor (search) to build nuclear weapons. Russia argues that cannot happen because the deal calls for spent fuel to be returned, and U.N. nuclear expian cooperation with the country would go ahead.

The head of Russia's Federal Atomic Energy Agency, Alexander Rumyantsev (search), is to sign an agreement Saturday on the supply and return of nuclear fuel, an official of Iran's Atomic Energy Organization, Ali Hafezi, told The Associated Press.

But diplomats in Vienna, Austria, cautioned Friday that Iran and Russia had often indicated a deal was imminent.

The agreement has been repeatedly delayed by what Iranian and Russian officials called technical and financial details. But diplomats in Vienna say U.S. pressure on Russia also has held it up.

Russia could be delivering nuclear fuel to Iran within two months of signing the deal, Rumyantsev spokesman Nikolai Shingarov told AP last week.

Russia helped build the $800 million reactor at Bushehr in southern Iran, and construction is now complete on the light water reactor capable of generating 1,000 megawatts of electricity. Rumyantsev is scheduled to visit the plant Sunday.

The United States, which accuses Iran of having a secret program to make nuclear weapons, had long urged Russia to terminate its assistance to Iran's nuclear development.

But U.S. officials have praised Russia for making its sale of fuel contingent on Iran's returning all of it. Experts say spent fuel from the Bushehr reactor could be used to produce enough plutonium to make 30 rudimentary atomic bombs a year.

Russia says such a danger will be avoided by ensuring all spent fuel is returned and by Iran's allowing officials of the International Atomic Energy Agency to monitor Bushehr.

Russia's ITAR-Tass news agency quoted the Russian ambassador to Iran, Alexander Maryasov, as saying that after the agreement is signed, "there will be no grounds to state that Iran may use nuclear fuel for other than peaceful purposes." Iran, which has agreed to IAEA monitoring at Bushehr, denies it seeks nuclear weapons and says its program aims only to generate electricity.

In Berlin on Friday, Iran's top nuclear negotiator Hassan Rowhani said he was hopeful Iran soon will be able to work out an agreement with Britain, France and Germany on his country's uranium enrichment program.

Iran has suspended the program, pending talks with the Europeans, but it is resisting international pressure to abandon enrichment. Iran insists it has the right to produce its own reactor fuel. Uranium enriched to a low degree is used for reactors, but it can be enriched further for nuclear bombs.

"We are confident that we will, through positive measures from all sides, see positive results in March," Rowhani said after talks with German Foreign Minister Joschka Fischer.

Fischer seemed less optimistic, telling reporters "the positions of the two sides are complex and difficult to bridge."

Tehran's chief nuclear negotiator, Hassan Rowhani, meanwhile, said in an interview published Friday in Paris' Le Monde newspaper that Iran might be hiding its nuclear technology inside special tunnels because of threats of attack by the United States.

Asked about reports of the tunnels, Rowhani said they "could be true."

"From the moment the Americans threaten to attack our nuclear sites, what are we to do? We have to put them somewhere," Rowhani said.

IAEA officials have said they are aware of the tunnels in the western city of Isfahan.