"As long as the Iranians have a sense that they are negotiating with the Europeans ad referendum (needing referral for a final decision), and what they discuss with them will have to be discussed with the Americans, and then come back again to them, I am not sure they will put everything on the table," Annan told reporters in Vienna.

European Union efforts since 2003 to win guarantees that Iran is not making nuclear weapons have foundered, with Iran pushing ahead since April on enriching uranium for what can be nuclear reactor fuel but also nuclear bomb material.

The United States has refused to talk directly to Iran but backs the EU diplomacy.

"I have asked all sides to lower their rhetoric and intensify diplomatic efforts to find a solution," Annan said.

"I have also stated very clearly both in private and in my contacts with the American administration and publicly that I think it is important that the United States come to the table and that they should join all the European countries and Iran to find a solution," he said on the sidelines of a European Union-Latin American summit.

On Wednesday the United States, which has failed to win support for UN sanctions against Iran, announced it would give its European allies "a couple of weeks" to draft a fresh approach to persuading Tehran to drop its disputed nuclear activities.

Diplomats said negotiators from the Security Council's permanent members -- the United States, Russia, China, Britain and France -- plus Germany planned to meet in London on May 19 to weigh a new package of incentives as well as penalties.

The United States charges that Iran is using a nuclear program it says is a peaceful effort to generate electricity to hide the development of nuclear weapons.

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.