South Korean President Lee Myung-Bak on Monday started a visit to the United States to coordinate action on North Korea, as one of his ministers said Pyongyang had been running a secret weapons program for years.

"South Korean President Lee Myung-Bak, seen here on June 6, 2009, speaks during a ceremony to mark the Korean Memorial Day at the National Cemetery in Seoul, South Korea. South Korean President Lee Myung-Bak on Monday started a visit to the United States to coordinate action on North Korea, as one of his ministers said Pyongyang had been running a secret weapons program for years."

Lee was due to meet later Monday with Secretary of State Hillary Clinton before a summit Tuesday with President Barack Obama, with the South Korean leader seeking reassurance of US commitments to defend the ally.

Speaking before his departure, Lee said he hoped to work with Obama on finding out how to stop North Korea after the hard-line communist state tested a nuclear bomb and stormed out of a six-nation disarmament accord.

Lee said that North Korea -- still technically at war with his country -- could proliferate nuclear technology to nations such as Iran and Syria which have rocky relations with the United States.

"If we are to assume that North Korea becomes a nuclear-power state, of course the danger of having an all-out nuclear war, that possibility is very slim," Lee told The Wall Street Journal in an interview.

"However, what really should concern us, and what concerns me, is the fact that North Korea nuclear capabilities may be used for nuclear terrorism," he said.

Lee, a conservative businessman, took office last year and -- to the delight of many in Washington -- reversed a decade-long "sunshine policy" under which Seoul provided aid to the impoverished North with few conditions.

"South Korea's 440-ton high-speed missile ship, the Yoon Young Ha (R), is followed by patrol boats as it sails in the Yellow Sea off the southwestern port of Pyeongtaek on June 15, 2009. The missile ship was deployed close the border two weeks ago amid high cross-border tensions following North Korea's nuclear test last month. AFP PHOTO/PARK YEONG-DAE"

In Seoul, Unification Minister Hyun In-Taek said that North Korea never intended to give up its atomic weaponry and is thought to have been developing a secret program for seven to eight years despite taking part in talks.

Responding defiantly to tougher United Nations sanctions following its nuclear test last month, the communist state vowed Saturday to build more bombs and to start a new weapons program based on uranium enrichment.

Hyun told a parliamentary hearing he believes the enrichment program -- a second route to an atomic bomb after the North's admitted plutonium operation -- had in fact been in existence for years.

"As the US raised the accusation in 2002, I believe (the uranium enrichment program) had started before that. I believe it has been there for at least seven to eight years," Hyun said in answer to a question.

The US claims in 2002, which were denied by the North, led to the collapse of a bilateral disarmament deal.

Six-nation talks began in 2003 and four years later reached an agreement which led the North to shut down the plants that make weapons-grade plutonium.

"An undated picture released by North Korea's official Korean Central News Agency on June 14, 2009 shows North Korean leader Kim Jong-Il inspecting the command of 7th Infantry Division of the Korean People's Army at undisclosed location in North Korea."

Amid US reports that North Korea could be preparing its third nuclear test, South Korea has sent extra troops and naval units to border islands seen as a likely flashpoint.

Lee will seek a written US commitment to provide a nuclear "umbrella" for South Korea in a summit joint statement, a Seoul presidential official told Yonhap news agency.

The North's ruling party newspaper Rodong Sinmun said Monday any such commitment would be "virtually formalizing a provocation for nuclear war."

It denounced Lee for an "intolerable" crime with his request for US nuclear protection, which would only help the peninsula become "a nuclear tinderbox."

The paper said North Korea's own nuclear program had deterred war on the peninsula, not the US umbrella.

The North's nuclear deterrent had served not only as a "merciless iron hammer" for aggressors but also as an "iron shield" for South Koreans, it said.

Several analysts and officials believe ailing leader Kim Jong-Il, 67, is intensifying military tensions to bolster his authority as he tries to put in place a succession plan involving his youngest son, Jong-Un.