"This is a very serious matter, the world knows it's a serious matter, and we're working together to solve this matter," he said on the sidelines of the Asia-Pacific Economic Cooperation (APEC) summit here.

"It's very important for the Iranian government to hear that we are concerned about their desires, and we're concerned about reports that show that prior to a certain international meeting, they're willing to speed up processing of materials that could lead to a nuclear weapon," Bush said.

Diplomats told AFP on Friday that Iran is producing the uranium feedstuff that could be used to make nuclear weapons, only days before it is due to introduce a promised ban on all such enrichment activities.

It was not clear if this would harm Iran's drive to convince the UN watchdog International Atomic Energy Agency (IAEA) not to send it to the UN Security Council, which could impose punishing sanctions, for what the United States says is a covert nuclear weapons program.

Bush was also expected to discuss Iran's nuclear ambitions in a midday meeting with Russian President Vladimir Putin.

The US State Department had said earlier in Washington that if reports that Iran was making the uranium hexafluoride (UF6) gas that is the feed used to make enriched uranium proved to be true, it would further erode Tehran's already shaky credibility on nuclear matters.

Enriched uranium, made by spinning UF6 in what can be cascades of thousands of centrifuges, can be fuel for nuclear reactors or the raw explosive material for atomic bombs.

Iran had agreed last Sunday to suspend all uranium enrichment activities, in an accord worked out with EU negotiators Britain, France and Germany in order to keep Iran from being sent to the Security Council when the IAEA's 35-nation board of governors meets in Vienna next Thursday.

"We appreciate the efforts of the governments of France, Germany and Great Britain to convince the Iranians to give up any nuclear ambitions they may have," Bush said after meeting with Japanese Prime Minister Junichiro Koizumi.

"And the reason why they're involved is because they do believe that Iran has got nuclear ambitions, as do we, as do many around the world," said the newly re-elected US president.

A total of "37 tonnes of uranium yellowcake ore have been introduced into Iran's uranium conversion facility in Isfahan, and an unknown amount of UF6 has been produced," a Western diplomat said.

The comments were confirmed by other diplomats close to the IAEA, which has inspectors in Iran ready to verify the suspension of uranium enrichment that Tehran has said will begin Monday.

Iran claims its nuclear program is a strictly peaceful effort to produce electricity and agreed to a full enrichment suspension as a confidence-building measure.

The United States has long charged that the oil-rich Islamic republic has no need for nuclear energy and that any atomic programs must be part of a secret effort to develop nuclear weapons.

Under IAEA investigation since February 2003 for nuclear activities it had hidden for almost two decades, Iran agreed in October 2003 to suspend the actual enrichment of uranium.

Under international pressure for continuing to make centrifuges, Iran agreed in February 2004 to suspend such support activities, but it said this did not include converting yellowcake into UF6, which is the first step in enrichment.

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.