Mr. Obama indicated firm resolve in how the international community will deal with the North, saying it poses "a grave threat." "We will pursue denuclearization on the Korean peninsula vigorously," he said at a press conference with Mr. Lee in the Rose Garden.
The United States has promised South Korea full nuclear protection since the Korean war. But Lee has wanted to secure written assurances of that protection, according to the South Korean press.
On Tuesday, the two countries released a joint statement that articulated the overall partnership between the two countries, including the nuclear element.
"We will maintain a robust defense posture," the statement read in part. "The continuing commitment of extended deterrence, including the U.S. nuclear umbrella, reinforces this assurance."
Tensions on the Korean peninsula have been ratcheting up. In response to a new United Nations Security Council resolution slapping tougher sanctions on North Korea for a recent nuclear test, Pyongyang announced Saturday that it would never give up its nuclear weapons, but would in fact set out to build more.
Both Obama and Lee indicated Tuesday that North Korea would not be able to do what it's done in the past: When it's waited long enough after a controversial act, the impoverished and isolated country has received concessionary loans and aid such as foodstuffs. North Korea "will not be able to repeat the past or their past tactics and strategies," Lee said at the press conference.
"We are going to break that pattern," Obama added.
The US is planning to begin enforcement of one provision of the UN resolution that authorizes countries to stop North Korean ships suspected of carrying banned cargo. The ships would be directed to a nearby port for inspection.
Tuesday's press conference also addressed the US-South Korea free trade agreement, which both countries signed two years ago. Final approval, however, has not happened, with passage in the US Senate in limbo over issues such as the number of Korean automobiles coming into the US. South Korea's tough quotas on US beef imports have also caused concern.
Lee has wanted to rekindle interest in final approval of the pact, and he indicated Tuesday new hopes to "chart our way forward on the agreement."
Obama was cool to new free-trade agreements on the presidential campaign trail, and he was specifically critical of the South Korea deal. But in the White House, he has called for passage of pending trade accords with South Korea, Panama, and Colombia, though with certain revisions in each case.
With North Korea dominating the chief executives' conversation, it was practically inevitable that the trade pact would take a back seat, North Asia analysts say.
At the press conference, Obama refrained from promising too much about the accord, but he did say, "I am committed to moving forward on a path that will increase commercial ties that are already very strong between our two countries."
Obama's approach to the issue speaks loudly about US leadership in the international economy and where he sees trade fitting into the economy's recovery, says Steven Schrage, an expert in international business at the Center for Strategic and International Studies in Washington.
Obama's words speak "not only about the US commitment to Korea and to the world's most dynamic economic region," he says, but also about "whether the US is in danger of, by inaction, abdicating its role as the leader of the movement pushing forward for trade and open economies, which will be so critical to the world's economic rebound."