WASHINGTON, Oct. 17 — President Bush warned today that Iran would be raising the risk of a “World War III” if it came to possess nuclear weapons.

And he said he believed that Russia still wanted to stop Iran from developing such weapons.

Those comments, made during a far-ranging 45-minute news conference, came as reporters sought the president’s reaction to a warning on Tuesday by President Vladimir V. Putin of Russia against any military strikes on Iran to halt the nuclear work that it has continued in defiance of much of the world. Iran contends that its nuclear program is purely peaceful.

“If Iran had a nuclear weapon, it’d be a dangerous threat to world peace,” Mr. Bush said. “So I 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.”

“I take the threat of Iran with a nuclear weapon very seriously,” he said.

The United States has said it is pursuing a diplomatic approach to Iran, including the threat of a new round of United Nations sanctions, but it has refused to rule out military action to halt Iran’s nuclear program.