The United States will reaffirm its longstanding pledge to use its nuclear deterrent to protect South Korea from attack by the North, officials said.
Shifting military relations between the two sides also are expected to figure prominently in the talks.
The officials will review the security situation on the Korean peninsula in the aftermath of an underground North Korean test October 9 that has confirmed Pyongyang's possession of nuclear weapons.
"We're very comfortable where all of our planning is right now. Our plans are in place. We have total agreement with our allies. So this is not an issue of concern," said a senior US defence official who briefed reporters Thursday on condition of anonymity.
"What is important to us is to restate and reaffirm the nuclear umbrella and, again, we are going to do it in exactly the same way we've done it in years past," the official said.
A South Korean official had said before the start of the consultations on Wednesday that Seoul wanted a more detailed understanding of the US nuclear umbrella.
But that idea was rebuffed by the US official, saying that there would be no public or private discussion of details beyond Washington's standard reaffirmation that it will use its nuclear umbrella to protect South Korea.
South Korea gave up its nuclear weapons in the 1970s and signed a 1992 agreement on the de-nuclearisation of Korean peninsula that the US official said still holds despite the North Korean test.
Rumsfeld, however, expressed concerns this week of the risk of rapid proliferation of nuclear weapons in the wake of the North Korean test, and officials have warned of the risk that Japan and South Korea might reconsider their positions.
Meanwhile, the two sides will discuss matters related to the relocation and consolidation of the 28,000 US troops in South Korea and a proposed change in the US-South Korean combined force command structure that would give Seoul control over its own forces in wartime.
Under the current arrangements, the combined force would come under a US general in the event of war.
The United States has said it is prepared to make the command change in 2009 but the South Koreans insist they will not be ready until 2012.

Copyright AFP (Agence France-Presse), 2006