After they met in the ancient Korean capital of Gyeongju, the two leaders said on Thursday that North Korea should "eliminate its nuclear weapons programmes promptly and verifiably".
"We reiterated that a nuclear-armed North Korea will not be tolerated and reaffirmed that the issue should be resolved through peaceful and democratic means," Roh said.
Bush gave no ground on Washington's position that North Korea will not get the light-water atomic reactor it wants for producing energy until it has verifiably dismantled its nuclear weapons and the programmes to make them.
"Our position is that we will consider the light-water reactor at the appropriate time. The appropriate time is after they have verifiably given up their nuclear weapons and/or programmes," said the US president.
The two leaders also recommitted themselves to six-country talks - with the United States, South Korea, Russia, China and Japan as partners in negotiations with North Korea - and Roh said a new round should come as soon as possible.
Roh, who in the past had resisted Bush's hardline approach to the issue, sidestepped a reporter's question about whether his policy, which emphasises reconciling North and South, put him at odds with Washington.
Bush is to attend APEC talks
which open on Friday
The US president acknowledged "complexities" in the bilateral relationship but stressed that ties between Washington and Seoul had "never been better" and emphasised his support for eventually reuniting the two Koreas.
"I see a peninsula one day that is united and at peace," said Bush. "There's a real possibility that by working together, at some point in time the peninsula will be united and at peace."
Bush also praised South Korean democracy and its economy, and thanked Roh for sending some 3000 troops in support of US efforts in Iraq - the third-largest contingent behind the United States and Britain.
Bush also lashed out at opposition Democrats who have stepped up charges that he twisted intelligence to exaggerate the threat posed by Saddam Hussein and convince the United States to go to war.
South Korea has some 3000
troops stationed in Iraq
Asked whether he sided with Vice-President Dick Cheney, who suggested that such criticisms undermine US forces on the front lines, or Republican Senator Chuck Hagel, who called asking questions in time of war a patriotic duty, Bush wasted no time in answering: "The Vice-President."
"I expect there to be criticism. But when Democrats say that I deliberately misled the Congress and the people, that's irresponsible," he said, as US polls showed more and more Americans doubt his honesty.
After a brief trip to Japan, Bush was in South Korea for talks with Roh as well as for the Asia-Pacific Economic Cooperation (APEC) forum summit, which opens on Friday. He was to travel to China and Mongolia before returning to Washington.
By the time he ends a week-long trip to Asia, Bush was to have consulted all the leaders of Washington's partners in the six-party North Korean talks.
Bush will meet Russian President
Putin during his Asia trip
He was to see Russian President Vladimir Putin and sit down in Beijing with Chinese President Hu Jintao.
Bush, who branded Pyongyang an outpost of "isolation, backwardness, and brutality" on Wednesday, had been expected to soften his rhetoric somewhat by expressing support for Roh's policy of reconciliation with the North.
At a previous round of talks in September, the parties issued a joint statement of principles in which North Korea promised to scrap its nuclear programmes in exchange for energy assistance and other benefits.
But a day later, North Korea insisted it would not dismantle its nuclear arsenal before the United States supplied it with a light-water atomic reactor to generate electricity.
The United States says North Korea must disarm first.
At last week's talks in Beijing, North Korea raised a new obstacle, accusing Washington of breaching the September agreement by imposing sanctions on its firms.