WASHINGTON - US President Barack Obama Tuesday vowed to defend South Korea and "vigorously" press North Korea to give up its nuclear weapons as the communist state's nuclear belligerence escalates.

South Korean President Lee Myung-Bak met with Obama at the White House after weeks of spiraling tension with North Korea which last month tested a nuclear bomb and is feared to be preparing another such experiment.

"We have reaffirmed the endurance of our alliance and America's commitment to the defense of the Republic of Korea," Obama said in a press appearance with Lee in the White House Rose Garden.

"We will pursue denuclearization on the Korean peninsula vigorously," Obama said.

He also said that the United States planned the "serious enforcement" of sanctions against North Korea, a week after the UN Security Council voted unanimously to tighten controls on arms shipments involving the hardline state.

Lee welcomed assurances that his country remains under the US security umbrella -- and said Obama made clear that it included the US nuclear deterrent.

"This has given the South Korean people a greater sense of security," Lee said.

"We agreed that under no circumstance are we going to allow North Korea to possess nuclear weapons," Lee said.

On one major issue for Lee, he said that he and Obama had agreed to working-level talks to advance a free trade agreement -- which the US leader had branded as "flawed" before taking office.

The two presidents were spending several hours together, including a private one-on-one session, expanded talks with officials, Obama's first Rose Garden press availability with a foreign leader and a working lunch.

Lee was then set to head to Capitol Hill for talks with the top Democratic and Republican leaders in the US Senate.

The summit comes a day after the latest show of defiance by North Korea, which said 100,000 people rallied to denounce a tightening of UN sanctions on the hardline communist state for testing a nuclear bomb.

Pyongyang sent east Asian tensions into overdrive last month with its second nuclear detonation, which followed what Washington said was a disguised test of a long-range missile in April.

Obama came to office offering negotiations with reclusive North Korea, but Kim Jong-Il's government has grown ever more defiant.

On Saturday, the North vowed to build more nuclear bombs and start enriching uranium for a new atomic weapons programme, in response to the new UN sanctions.

Some analysts have speculated that the saber-rattling is primarily rooted in an attempt by ailing 67-year-old Kim to bolster a succession plan involving his youngest son, Kim Jong-Un.

The United States stations some 28,500 troops in South Korea and more than 40,000 more in nearby Japan, which also has tense relations with Pyongyang.

Lee, a conservative businessman, took over last year and delighted many in what was then George W. Bush's Washington by reversing a decade-long "sunshine policy" under which South Korea put few restrictions on aid to the impoverished North.

The latest nuclear showdown with North Korea is clouding the fate of two female US journalists jailed by Pyongyang last week for 12 years.

Official North Korean media said Tuesday that Laura Ling, 32, and Euna Lee, 36, had confessed to a politically motivated smear campaign.

Washington has said it considers the case of the reporters as separate from the nuclear row and has been trying to win their release.

There has been growing speculation in Washington meanwhile that North Korea may soon conduct its third nuclear test, despite a unanimous vote by the United Nations Security Council last week to tighten sanctions on Pyongyang.

Media reports from Seoul also detailed how the North finished preparatory work for a new launch pad for long-range missiles on its northwest coast.

Satellite photos of the Dongchang-ri site reportedly showed the structure to be capable of firing an intercontinental ballistic missile.