The U.S. is pledging "immediate support" of South Korea as part of a "firm" commitment to defend its ally under America's nuclear umbrella, stronger language than in last year's version of their long-standing policy statement.

Noting North Korea's nuclear test Oct. 9, Defense Secretary Donald H. Rumsfeld and South Korean Defense Minister Yoon Kwang-ung "demanded that North Korea refrain from any further action that might aggravate tensions," according to the joint communique made public Saturday.

Both sides agreed that North Korea's "continued development" of weapons of mass destruction and long-range missiles, and the threat of proliferation were "a challenge" to the alliance between the United States and the Republic of Korea, the defense chiefs said in the statement.

It said Rumsfeld "offered assurances of firm U.S. commitment and immediate support" to South Korea, including continuation of the extended deterrence offered by the protective umbrella of the U.S. nuclear arsenal.

By comparison, last year's statement said Rumsfeld "reaffirmed the U.S. commitment to the security" of South Korea and "to the continued provision of a nuclear umbrella" for the U.S. ally.

At their meeting Friday, Rumsfeld and Yoon disagreed over how explicitly the U.S. should say that South Korea is under that umbrella.

The recent North Korean nuclear test has raised South Koreans' concerns about the future of their defense relationship with the United States. Already, there are significant changes in the works as the U.S. reduces the number of its ground troops in South Korea and pulls troops farther from the North Korean border area.

The South Korean delegation sought to persuade Pentagon officials to issue a public statement saying that if the North attacked the South with nuclear weapons, the U.S. would retaliate as if American territory itself had been attacked by nuclear weapons.

U.S. officials resisted, saying that would go beyond what the U.S. normally says publicly about how it would respond with regard to nuclear weapons. For decades, U.S. policy has been deliberately designed to include some ambiguity about the circumstances under which it would use nuclear arms.

Japan is covered by a similar U.S. "umbrella" promise.

The new joint statement said Yoon "expressed appreciation for the close cooperation and steadfast support of the U.S. in the face of North Korean intransigence."

Rumsfeld, at a news conference Friday with Yoon, noted that a public statement is issued each year following the U.S.-South Korean defense talks and that it always reaffirms the provision of a nuclear umbrella for South Korea, consistent with the 1954 U.S.-South Korean defense treaty.

Rumsfeld said he was unaware of any proposal to change the language, "nor can I imagine how it could be improved upon."

Yoon responded by saying the matter had been discussed extensively in a series of meetings this past week and that he hoped the language on U.S. nuclear protection for South Korea would be different than in years past.