President Bush says world leaders risk bringing about World War Three if they do not do more to prevent Iran from getting nuclear weapons.

Mr. Bush said at a White House news conference Wednesday that he takes the threat of a nuclear-armed Iran very seriously. He noted that Iranian President Mahmoud Ahmadinejad has said Israel should be destroyed.

Mr. Bush said the United States will continue to push for tighter economic sanctions on Iran. He said that those he calls "responsible folks" inside Iran will get tired of isolation and decide Iran's current policy is not worth it.

After meeting with President Ahmadinejad in Iran earlier this week, Russian President Vladimir Putin reaffirmed his support for Iran's nuclear energy program. He has also said he sees no evidence Iran is looking to build a nuclear bomb.

Mr. Bush said Wednesday he looks forward to having Mr. Putin clarify his remarks. He said Mr. Putin told him last month at the APEC summit in Australia that the world must ensure that Iran does not have the capacity to build a nuclear weapon.

Iran's chef nuclear negotiator Ali Larijani said Wednesday Mr. Putin has put forward a proposal regarding Iran's nuclear program during talks with Supreme leader Ayatollah Ali Khamenei. Larajani did not elaborate.