WASHINGTON (AP) _ A top aide to Russian President Vladimir Putin denied that Russia was helping Iran's nuclear weapons program. He called on the United States Monday to respond to Russia's proposal for negotiations to reduce U.S. and Russian nuclear arsenals.

Igor Sergeyev, who advises Putin on strategic issues, said the technology Russia provides to Iran is for use in a light water reactor, much like technology the United States is giving to North Korea under an agreement to freeze that country's nuclear weapons program.

``To obtain weapons from the light water reactor in Iran is impossible,'' he said at a conference on proliferation problems held by the Carnegie Endowment for International Peace. Sergeyev acknowledged that controls on technology exports beyond the range of the Russian government had been a ``headache'' in the early 1990s.

But he said the list of prohibited materials for export has grown through the years.

At the same time, Sergeyev said the United States had failed to respond to proposals by Putin to place a ceiling of 1,500 on U.S. and Russian long-range nuclear warheads.

``It's paramount to start negotiations immediately,'' he said.

The Russian official said he found hope in a general willingness of the Bush administration to reduce stockpiles.

At the same time, he defended the 1972 Anti-Ballistic Missile Treaty, which Bush wants to abandon to make way for a U.S. shield against missiles.

The spread of sophisticated technology has become more dangerous, he said.

``The world may be entering a phase in which the use of nuclear weapons is more likely than before,'' the former Russian defense minister said.

In fact, he said, Russia is more vulnerable than the United States to theft of nuclear technology and accidental launches of missiles by other nations.

``We are hoping to improve our export control,'' he said. ``It is one of the best control systems.''