President Bush said Saturday that he believes Iran is continuing to pursue a nuclear weapon, which he called "a very serious matter," and said he had won pledges from Asian allies to increase pressure on North Korea's leader to restart disarmament talks.

During his reelection campaign, Bush said little about the two nuclear threats. But with aides contending that his victory gave him new international leverage, he took confrontational lines with both countries, insisting they disarm but pledging to pursue that goal diplomatically.

At his first international summit since being reelected, and on his first trip abroad in five months, Bush escalated warnings issued by outgoing Secretary of State Colin L. Powell, saying he did not believe claims by Iran's ruling clerics, who have denied that the country was taking steps to develop a nuclear weapon.

Bush said the United States was closely monitoring Iran's activities in the run-up to Thursday, when the International Atomic Energy Agency is scheduled to meet in Vienna to determine whether to refer the country's nuclear activity to the U.N. Security Council. In an agreement with Britain, France and Germany that was announced this month, Iran agreed to suspend uranium enrichment, the pivotal process in a peaceful nuclear energy program capable of being diverted for military use.

"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 on the sidelines of the 12th annual Asia-Pacific Economic Cooperation (APEC) summit. "The world knows it's a serious matter, and we're working together to solve this matter."

Bush said it was "very important for the Iranian government to hear that we are concerned about their desires," and that he would continue working with European powers "to convince the Iranians to give up any nuclear ambitions they may have."

"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," Bush said, as Japanese Prime Minister Junichiro Koizumi stood at his side.

The president met with the leaders of his four partners in arms talks with North Korea -- China, Japan, Russia and South Korea -- and said afterward that all had supported his call for them to bolster their united front to try to get North Korea to return to talks that have been on hold since June. Bush went into the summit determined to urge them to more energetically apply pressure on North Korea, and by day's end, he had declared his diplomacy a success.

In a speech to chief executives meeting here at the base of the Andes Mountains, Bush said: "I can report to you today, having visited with the other nations involved in that collaborative effort, that the will is strong, that the effort is united and the message is clear to Mr. Kim Jong Il: Get rid of your nuclear weapons programs."

A Chinese Foreign Ministry official told reporters that President Hu Jintao had said to Bush, "This is a rather complex issue, and it requires all relevant parties to display patience, flexibility and sincerity."

Bush ate lunch with Russian President Vladimir Putin, and in contrast to their previously jovial appearances together, the two men were silent when photographers were briefly ushered in. A senior administration official who attended the meeting said Bush expressed skepticism about Putin's proposals to change the Russian political system in ways that have raised questions about his commitment to democracy.

The official said Bush "noted the concerns that we've had about checks and balances, about the centralization of power inside Russia, and asked Putin to give his own explanation of what was going on and why these steps were being taken inside Russia."

The official said Putin "went back deep into Russian history, the Stalinist period, and made the point that what the Russian government was trying to do at this point was to develop a democratic style of government that was consistent with Russian history and the unique problems that Russia faced as a multiethnic society on a large landmass."

Bush branded Iran part of an "axis of evil" in 2002, along with Iraq and North Korea, and in April said it would be "intolerable" if Iran were to develop an atomic weapon. Iran agreed Monday to a deal brokered by three European powers -- Britain, France and Germany -- to indefinitely suspend uranium enrichment until a permanent agreement could be reached to ensure that Tehran complies with its obligations as a signatory to the nuclear Non-Proliferation Treaty. But diplomats in Vienna have reported that Iran is scrambling to convert nearly raw uranium, also called yellowcake, into hexafluoride gas, the end stage for the uranium before it can be enriched, in advance of the deadline.

Powell, appearing a few hours after Bush at a joint press conference with Chile's foreign minister, Ignacio Walker, expressed frustration with Iran over what he depicted as its clandestine efforts to develop a nuclear weapon.

"Iran has been working on long-range missiles," Powell said. "They have been working on intercontinental range missiles, which they claim are for perhaps space-launch purposes. And we have reason to believe that when you see what they have been doing, the high aspects of their nuclear programs, when you see what they have been doing over the years with missiles and potential delivery systems, it is a cause of concern."

Speaking about North Korea after his meeting with Koizumi, Bush said it was "very important for the leader of North Korea to understand that the six-party talks will be the framework in which we continue to discuss the mutual goal we all have, which is to rid the Korean Peninsula of nuclear weapons."

A Japanese government official said after the meeting that Koizumi had told Bush he wanted "to continue cooperation toward Iraq's reconstruction," but stopped short of promising to extend the deployment of Japanese troops beyond their current commitment of Dec. 14.

Bush met with Hu, China's president, for the first time since Hu assumed his full powers. "I invited President Hu to come and visit the United States as soon as he can, and he invited me to China," Bush said.

During Bush's meeting with the chief executives, he won the heartiest applause when he recognized "a man who has served our country so well, a great United States secretary of state, Colin Powell."

"Right after my speech, he's headed to the Middle East. That's a heck of a retirement, Mr. Secretary," Bush said, drawing laughter. "I look forward to your report when you get back."

Bush, reprising an issue he had discussed with Koizumi, acknowledged to the leaders his "concern about whether or not our government is dedicated to dealing with our deficits."

He said he looked forward to outlining to Congress in his State of the Union address in January the steps he will take in his new budget to deal with the deficit.

Chilean President Ricardo Lagos and President Bush arrive at Santiago's Espacio Riesco conference center for APEC's annual summit.