By Matt Spetalnick

WASHINGTON, Oct 17 (Reuters) - U.S. President George W. Bush warned on Wednesday a nuclear-armed Iran could lead to World War III as he tried to shore up international opposition to Tehran amid Russian skepticism over its nuclear ambitions.

Bush was speaking a day after Russian President Vladimir Putin, who has resisted Western pressure to toughen his stance over Iran's nuclear program, made clear on a visit to Tehran that Russia would not accept any military action against Iran.

At a White House news conference, Bush expressed hope Putin would brief him on his talks in Tehran and said he would ask him to clarify recent remarks on Iran's nuclear activities.

Putin said last week that Russia, which is building Iran's first atomic power plant, would "proceed from the position" that Tehran had no plans to develop nuclear weapons but he shared international concerns that its nuclear programs "should be as transparent as possible."

"The thing I'm interested in is whether or not he continues to harbor the same concerns that I do," Bush said. "When we were in Australia (in September), he reconfirmed to me that he recognizes it's not in the world's interest for Iran to have the capacity to make a nuclear weapon."

Bush, who has insisted he wants a diplomatic solution to the Iranian issue, is pushing for a third round of U.N. sanctions against Iran.

Russia, a veto-holding member of the Security Council, backed two sets of limited U.N. sanctions against Iran but has resisted any tough new measures.

Stepping up his rhetoric, Bush said a nuclear-armed Iran would pose a "dangerous threat to world peace."

"We've got a leader in Iran who has announced that he wants to destroy Israel," he said. "So I've told people that, if you're interested in avoiding World War III, it seems like you ought to be interested in preventing them from having the knowledge necessary to make a nuclear weapon."

PUTIN'S "SPECIAL MESSAGE"

Iran rejects accusations it is seeking to develop a nuclear bomb, saying it wants nuclear technology for peaceful civilian purposes such as power generation, and has refused to heed U.N. Security Council demands to halt sensitive uranium enrichment.

Chief Iranian nuclear negotiator Ali Larijani was quoted by Iran's official IRNA news agency on Wednesday as saying that Putin had delivered a "special message" on its atomic program and other issues. No other details were given.

Putin's visit on Tuesday was watched closely because of Moscow's possible leverage in the Islamic Republic's nuclear standoff with the West. It was the first time a Kremlin chief went to Iran since Josef Stalin in 1943.

Asked about Putin's "special message," U.S. State Department spokesman Tom Casey said he was not aware of any deal or offer put forward by Moscow to Tehran over the nuclear program.

On Russian opposition to Caspian Sea states being used to launch attacks against Iran, Casey reiterated that Bush kept all his options on the table but that the United States was committed to the diplomatic path with Tehran. (Additional reporting by Frederick Dahl in Tehran and Sue Pleming in Washington)

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