ST. PETERSBURG - U.S. President George W. Bush and Russian President Vladimir Putin tried to put aside differences over Iraq on Sunday, looking instead to cooperation on thwarting the spread of illicit weapons in Iran and North Korea.

Even so, the two leaders remained at least partly at odds over Russia's technology assistance to Iran - help the United States claims is allowing Tehran to speed development of nuclear weapons. Iran says its nuclear program is strictly for peaceful development of energy.

Putin said he needed no convincing that nuclear and other mass-killing weapons must be "checked and prevented throughout the world" in Iran and elsewhere.

But, he added, "We are against using the pretext of a nuclear weapons program of Iran as an instrument of unfair competition against us."

It was a clear reference to U.S. sanctions, and the threat of additional sanctions, against Russian companies with certain business dealings with Iran.

"Sometimes - and we have encountered this - baseless pretensions were made toward Russian companies about their cooperation with Iran," Russian Foreign Minister Igor Ivanov said later. "We do speak out against such dishonorable competition and we will continue to speak out against it. "

It was a rare jarring note at an otherwise harmonious meeting - their first since the Iraq war, which Putin opposed. Bush invited Putin to the presidential retreat at Camp David this September.

At a joint news conference, they pledged cooperation in working to curb the nuclear ambitions of Pyongyang and Tehran, in fighting terrorism and the spread of lethal weapons and in rebuilding Iraq.

"We strongly urge North Korea to visibly, verifiably and irreversibly dismantle its nuclear weapons program," Bush said. "We are concerned about Iran's advanced nuclear program and urge Iran to comply in full with its obligations under the Nuclear Nonproliferation Treaty."

Bush called Putin "Vladimir" and "my good friend." "Our relationship is broad," Bush said. He seemed impatient at times, though, tapping his hand on his massive desk during the news conference.

On his third visit to Putin's hometown, Bush joined other world leaders in helping to celebrate St. Petersburg's 300th anniversary.

Bush and Putin celebrated in signing papers acknowledging that the legislatures of both countries had ratified the biggest single nuclear-arms reduction treaty in the history of the two powers, reducing their respective arsenals by two-thirds.

The U.S. Senate ratified the "Treaty of Moscow" earlier this year, the Russian parliament last month.

Putin even agreed to work more closely with Bush on developing missile-defense systems, completing a full 180-degree turn from his bitter 2001 opposition to Bush's plans to build a U.S. missile defense program - a move that resulted in the scrapping of a landmark 1972 nuclear-arms agreement.

In his most conciliatory words yet since his vocal opposition to the U.S. led war on Iraq, Putin said he was committed to working "in practical terms" with the United States on postwar issues. He said he fully agreed with Bush on the need for the Iraqi people to determine their own future.

"Cooperation will continue to expand and develop," Putin said.

Putin made a bid for Russian oil and other companies to bid for business deals in postwar Iraq. "Russian companies have a wealth of experience operating in Iraq," he said.

Bush said he believed the Russian leader was "committed to working for a sustainable democracy in Russia where human, political and civil rights will be fully ensured." The U.S.-Russian disagreement over going to war in Iraq "will make our relationship stronger, not weaker," Bush said.

Bush's efforts to mend fences with Putin came before he met with another Iraq war critic, French President Jacques Chirac, host of this year's Group of Eight summit of major industrial powers in Evian, France. However, Bush's differences with Chirac seemed to be more deep-seated than those with Putin.

In the lead-up to war, France, Russia and Germany had maintained that U.N. weapons inspections should have continued in Iraq and that diplomatic options had not been exhausted.