In return, South Korea will expand its support for the U.S.-led mission in Afghanistan, possibly dispatching police or military troops to help stabilize the war-torn country.
The agreement came Thursday at an annual security consultative meeting in Seoul between U.S. Secretary of Defense Robert Gates and South Korean Defense Minister Kim Tae-young.
Gates "reaffirmed the U.S. commitment to provide extended deterrence for the (Republic of Korea), using the full range of military capabilities, to include the U.S. nuclear umbrella, conventional strike and missile capabilities," said a joint statement issued at the end of the talks.
In June, U.S. President Barack Obama promised to extend the U.S. nuclear umbrella "wide enough to protect" the South in the first written guarantee by a U.S. president, in response to a debate as to whether South Korea should be armed with nuclear weapons, fueled by North Korea's May nuclear test and missile launches in April.
Under the nuclear umbrella, the United States could launch a nuclear strike on North Korea in case of atomic attack. The United States has also provided the nuclear umbrella to its other key Asian ally, Japan.
To bolster the nuclear umbrella, the United States will use its regional missile defense network, which can intercept projectiles from North Korea. In addition, the United States will mobilize its military assets to their maximum capacity if needed to defend South Korea against North Korea, Gates said in the statement.
"In addition to the traditional military threat, North Korea's ballistic missiles and emerging nuclear programs have a destabilizing effect both regionally and internationally," Gates told a news conference following three hours of talks with Kim.
"We will stand together with the Republic of Korea and our other allies and partners toward achieving the complete, verifiable denuclearization of North Korea," he said.
The South's defense chief said Seoul and U.S.-led allies are examining "all possible scenarios" that may arise in North Korea.
"We agreed to strengthen cooperation to cope with all possible scenarios to make sure there are no negative effects as a result of instability in North Korea," Kim said.
The agreement is likely to trigger an angry response from North Korea. The country has demanded that the United States withdraw its nuclear umbrella from South Korea, calling it an attempt to invade the country with atomic bombs and evidence that Washington is hostile toward it.
The North has also called for a withdrawal of U.S. troops from South Korea and has demanded a non-aggression or peace treaty with Washington to end U.S. military presence on the peninsula. The United States has 28,500 troops stationed in South Korea as a deterrent against North Korea under a mutual defense treaty signed just after the 1950-53 Korean War.
Backed by the U.S. troops, the South's 655,000 troops are facing off against the North's 1.2 million-strong armed forces. The Korean peninsula remains technically in a state of war as the conflict ended without a peace treaty.
"The U.S. commitment to provide extended deterrence would help ease security jitters in South Korea," said Baek Seung-joo, a senior analyst at Seoul's state-run Korea Institute for Defense Analyses.
In return for the U.S. commitment, South Korea promised to play a bigger role in the U.S.-led operations in Afghanistan, beyond the medical and job-training assistance that Seoul is already supplying in the country.
Gates said he had made "no specific request during my visit here in terms of Afghanistan," apparently to avoid a possible public outcry in South Korea.
"It is entirely up to the government of the Republic of Korea what it chooses to do in Afghanistan," he said.
But Gates indirectly called for a bigger contribution from Seoul, saying South Korea's international military contributions "should be seen as what they are -- something that is done to benefit your own security and vital interests."
He added, "There are a range of needs from helping to pay for the expansions and sustainment of the Afghan army and police."
In response, Seoul is considering dispatching some 300 police or military troops, government sources said. But the government is hesitating to make an open decision due to public concerns that South Koreans could be the target of terror attacks.
In July 2007, a group of 23 traveling South Koreans were seized by Taliban militants, who blasted Seoul's participation in U.S.-led coalition forces in Afghanistan fighting against Taliban insurgents. Two of the hostages were killed before South Korea withdrew its troops from Afghanistan in December 2007, ending its six-year military presence.
In March, four South Korean tourists were killed and three others injured in what appeared to be a terrorist bomb attack in Yemen, where al-Qaida has a strong presence. In June 2004, a South Korean worker in Iraq was held hostage and killed by Islamic insurgents after Seoul refused to bow to their demands to pull its troops out of Iraq.
South Korea sent 3,600 troops to Iraq in 2004 as the United States' second-largest coalition partner after Britain in the U.S.-led war against terrorism. It withdrew all of the troops last December.