Annual US-South Korean defence talks will conclude on Friday with a final communiqué containing the same language on the US commitment that it has every year since 1978, said the official, who spoke on condition of anonymity.
"We have really no plans to discuss the details of the umbrella either publicly or privately," said the official.
He said the US commitment on nuclear deterrence "simply is a fact. It's a component of our mutual defence treaty. And we're happy to let the statements stand for themselves as they have for the past 30 odd years."
The official said he was addressing the issue because of speculation in South Korea and the region in the wake of North Korea's nuclear test October 9.
Secretary of State Condoleezza Rice was in Seoul Thursday to discuss how to respond to North Korea's nuclear test.
US Defence Secretary Donald Rumsfeld meets here on Friday with South Korean Defence Minister Yoon Kwang-ung in the final session of the annual military talks.
Before the talks began, a South Korean official was reported to have said both sides will try to spell out the US nuclear umbrella pledge in detail.
Seoul was considering asking Washington, which withdrew all its nuclear weapons from the peninsula in 1991, to elaborate on what types of tactical nuclear weapons can be provided, the official said.
The US official said the South Koreans had not pressed for a more detailed pledge.
South Korea gave up its quest for nuclear weapons in the 1970s under strong US pressure.
The US official said Washington assumed Seoul would stick by its commitment not to pursue its own nuclear weapons capability, which he said was codified in the 1992 agreement on de-nuclearizing the Korean peninsula.
"The suggestion has been that the North Koreans have let's say torn the agreement up," the official said.
"The South Korean government continues to state that it is bound by and will continue to adhere to that agreement and considers it to be still in force, which is a very important point of departure for us to engage with the North Koreans in the future," he said.
Rumsfeld warned on Tuesday, however, of the risk of a rapid spread of nuclear weapons to other states if the international community fails to stop North Korea and Iran's nuclear programs.
The senior US defence official sought to play down South Korean reports that talks would cover changes in contingency planning to reflect that North Korea has nuclear weapons.
He said the alliance was already so close "really means there is nothing new here to be planned, as far as wartime planning goes."
"We're very comfortable with where all of our planning is right now. Our plans are in place. We have total agreement with all of our allies. So this is not an issue of concern," he said.
On another issue, the official said the two sides were still at odds over the timing of a plan to transfer to the South Koreans wartime control of their troops.
The US side wants it to happen in 2009, whereas the South Koreans say they will not be ready until 2012.

Copyright AFP (Agence France-Presse), 2006