The warning came after a two-day summit between US leader George W. Bush and Russia's Vladimir Putin dominated by nuclear concerns over Iran and North Korea -- two members of Bush's so-called "Axis of Evil" and US attempts to secure international help in Iraq.
But the summit at the Camp David presidential retreat ended with signs of an increasingly strong relationship between the leaders, despite their differences.
Putin, whose country is helping Iran to build a nuclear reactor, told a joint press conference that a "clear but respectful signal" should be sent to Tehran about the need to cooperate with the International Atomic Energy Agency (IAEA) over its nuclear facilities.
Putin went on that "Russia has no desire and no plans to contribute in any way to the creation of weapons of mass destruction, either in Iran or any other spot or region in the world."
The United States has accused Iran of seeking nuclear weapons and is pressing Russia not to complete an accord on providing the fuel that would allow Iran's first nuclear power plant to go online in 2005.
The IAEA has set an October 31 deadline for Iran to account for its nuclear activities. But the agency has postponed sending a monitoring mission that was to leave Sunday, to give Iran more time to prepare.
Bush said Russia and the United States "share a common goal, and that is to make sure Iran doesn't have a nuclear weapon, or a nuclear weapons programme."
"We also understand that we need to work together to convince Iran to abandon any ambition she may have," Bush added.
The United States says North Korea already has nuclear weapons, but Putin said new diplomatic efforts must be made to entice Pyongyang to end its weapons drive.
Putin said a "priority" must now be made of defusing the heightened tensions on the Korean peninsula caused by the new nuclear crisis and creating "a favourable atmosphere for a constructive dialogue."
He said "Russia believes that ensuring nuclear nonproliferation should be accompanied by extending to North Korea guarantees in the security sphere. We intend to continue our joint work with the United States in resolving this issue."
The United States has so far been reluctant to make a non-aggression pact with the North. But Russia and the United States were among the five nations that held talks with North Korea in Beijing last month on its nuclear programme.
Putin said Russia's contribution to policing and rebuilding Iraq could only be decided after a new UN resolution is passed, which sets out a United Nations role in the war-stricken country now run by a US-led coalition.
The Russian leader insisted his country wants to see Iraq normalised "as soon as possible."
He added: "At the same time, we understand it is a complicated process that should be based on a solid legal and administrative basis and should go ahead stage by stage.
"The degree and extent and level of Russia's participation in the restoration of Iraq will be determined when we know the parameters of the new resolution on Iraq," Putin told the press conference.
Bush insisted he was still "pleased" with the level of international help he is getting in Iraq.
"I recognize that some countries are inhibited from participation because of the lack of a UN resolution. We are working to get a satisfactory resolution out of the United Nations. We spent some time talking about that today."
Bush highlighted the high degree of "trust" he has built with the Russian leader despite differences they have had, particularly over Iraq.
The US leader was careful to call for an end to hostilities and terrorist attacks in Chechnya.
"Terrorists must be opposed wherever they spread chaos and destruction, including Chechnya," Bush said.
"Vladimir and I had very frank discussions about Iraq. I understood his position. He understood mine, but because we've got a trustworthy relationship, we're able to move beyond any disagreement over a single issue. Plus, I like him. He's a good fellow to spend quality time with," the US leader declared.
Putin replied: "We had differences over Iraq, in terms of practical ways how to resolve this problem, but we had understanding on the essence of this problem."