"We expect the two leaders to present a very united front. They both are urging North Korea and its leaders to engage with the international community and give up their pursuit of nuclear weapons," he said.
"Over the last few months there have been disagreements between South Korea and the US over how to approach this.
"But it seems the two leaders are now together in their approach."
Washington announced last week that it would send a senior envoy to North Korea for direct bilateral talks on the country's nuclear programme.
The US has said it is willing to sit down with North Korea if such a meeting is considered part of the six-nation talks that led to the 2005 and 2007 agreements for the North to give up its nuclear weapons programme in return for aid and other concessions.
The two leaders were also expected to discuss a free trade agreement (FTA) that was signed between the US and South Korea in 2007, but is yet to be ratified in the US Congress or in Seoul.
"We're going to be discussing this with South Korea. I want to get the deal done," Obama said in an interview with Fox News on Wednesday.
Seoul has appeared increasingly impatient at US efforts to renegotiate parts of the FTA, which would be the biggest US trade pact since the North American Free Trade Agreement was signed in 1994.
"We hope President Obama will express a more aggressive position on the FTA and are working towards that end," Lee's spokeswoman said earlier this month.
As a presidential candidate Obama termed the pact "deeply flawed" and his administration has since indicated it wants changes to open South Korea wider to US beef and auto exporters.
But on Wednesday, he said: "Overall, I think it's a potential good deal for US exporters."
Few concrete deals
Obama arrived in South Korea after a visit to China, where he and Hu Jintao, China's president, promised to work together on the economy, climate change and North Korea.
Before leaving for Seoul on Wednesday, the US president met Wen Jiabao, China's premier, who told Obama that his country wished to balance the flow of trade with Washington.
"China does not pursue a trade surplus," Wen said, adding that his government wants "to encourage a steady balancing of bilateral trade".
"Lively global trade and investment will help to overcome the international financial crisis and accelerate global economic recovery," he said, while urging both countries to "together oppose trade and investment protectionism".
After his talks with Wen, Obama wrapped up his trip to China by visiting the Great Wall.
Some analysts believe Obama went into the meetings with a weaker hand than most past US presidents, with US prestige battered by the economic recession as well as the wars in Iraq and Afghanistan, rubbing up against a China that is rapidly growing and gaining global clout, not to mention holding $800bn of US debt.