US President Barack Obama said he was willing to help North Korea repair its economy and end decades of international isolation if Pyongyang stopped a cycle of threats and finally moved toward nuclear disarmament.

Speaking to reporters at the end a week-long Asia tour, Obama said he and South Korean President Lee Myung-bak agreed the North must end a pattern of provocations that simply ended up with demands for more concessions but never resolved the central problem.

The two leaders also pledged to work to secure approval by their respective legislatures for their own free trade deal, signed more than two years ago. Lee said he was ready to discuss the main issue holding up ratification -- opening up the auto market.

"Our message is clear. If North Korea is prepared to take concrete and irreversible steps to fulfill its obligations and eliminate its nuclear weapons program, the United States will support economic assistance and help promote its full integration into the community of nations," Obama said.

The talks were among the least problematic for Obama on his Asian tour that started in Japan, where divisions remain over the location of a US military base, and also took in China, where he barely bridged divides on trade, currency policy and Tibet.

Obama and Lee have piled pressure on the destitute North by targeting its finances and telling Pyongyang it will win massive rewards if it abandons its atomic ambitions.

"The thing I want to emphasize is that President Lee and I both agree we want to break the pattern that existed in the past, in which North Korea behaves in a provocative fashion, and then is willing to return to talk ... and then that leads to seeking further concessions," Obama said.

He said he would send his first envoy to North Korea on December 8 to press Pyongyang to return to talks, frozen for almost a year, with regional powers to give up building a nuclear arsenal.

Analysts said he would not have agreed to it if he was not sure Pyongyang would reciprocate by returning to talks.

North Korea rattled the economically powerful region just ahead of Obama's first visit to Seoul since taking office by sparking a naval fight with the South and telling the world early this month it had produced a fresh batch of arms-grade plutonium.

But it has toned down its normally strident anti-US rhetoric since Obama began his trip to Asia a week ago. Lee said he held out hopes for a deal with the reclusive neighbor under which it would end its decades-long ambitions to build a nuclear arsenal.

"I hope that by accepting our proposal, the North will secure safety for itself, improve the quality of life for its people, and open the path to a new future," Lee said.

Thousands of cheering South Koreans lined the streets of downtown Seoul as Obama's motorcade drove by.

Police stopped one group of anti-Pyongyang protesters setting fire to a North Korean flag as the Obama convoy drove through the center of Seoul, barely 40 km (25 miles) from the border that has divided the two Koreas for more than half a century.