Responding on Monday to reports that Pyongyang launched a short-range missile into the Sea of Japan on Sunday, Rice said: "I don't think there should be any doubt about our ability to deter whatever the North Koreans are up to."
And in reassuring South Korea, Japan and other allies in the Pacific area, Rice said: "This is not just between the United States and North Korea."
A suggestion on Thursday by Vice-Admiral Lowell Jacoby, director of the US Defence Intelligence Agency, that North Korea might be able to strike US territory with a nuclear-tipped missile also has raised tensions and concerns.
A South Korean government official said US satellite images showed brisk construction activities at Kilchu in North Hamkyong Province, 350km northeast of the capital Pyongyang, the South Korean newspaper Chosun Ilbo reported.
"Synthesising satellite photos and other information, US intelligence authorities concluded North Korea may be preparing a nuclear test in the area and conveyed related photos and analyses to our intelligence authorities," the government official said.
"This is not just between the United States and North Korea"
US Secretary of State
But South Korea's Defence Ministry on Tuesday denied a report that US intelligence officials told Seoul that North Korea might be preparing a nuclear test.
Negotiations with North Korea on its nuclear weapons programme have reached a stalemate. Rice said North Korea's missile programme should be put on the agenda when and if six-party talks are resumed.
The US, Japan, China, South Korea and Russia hope to negotiate an end to North Korea's nuclear programme in exchange for security assurances and economic benefits.
Talks were supposed to resume last September, but North Korea withdrew its promise to attend. Since then, the North and the US have been exchanging angry rhetoric.