"There should be no doubt: we will draw upon the full range of our capabilities to protect against, and to respond to, the threat posed to us and to our allies by North Korea," Donilon said in remarks prepared for delivery distributed by the White House.
"This includes not only any North Korean use of weapons of mass destruction but also, as the president made clear, their transfer of nuclear weapons or nuclear materials to other states or non-state entities," he said. "Such actions would be considered a grave threat to the United States and our allies and we will hold North Korea fully accountable for the consequences."
Donilon also said Obama will meet with newly inaugurated South Korean President Park Geun-hye at the White House in May.
The South has tried to boost public confidence of by issuing stern warnings of its own. Defense Ministry spokesman Kim Min-seok said last week that North Korea's government would "evaporate from the face of the Earth" if it ever used a nuclear weapon.
Despite North Korea's threat of nuclear strikes on Washington, experts believe Pyongyang still lacks the technology to create a miniaturized warhead to place on a ballistic missile. But with the South Korean capital of Seoul just 40 miles south of the border and the North's million-man army, the risks for the South are more acute.
State Department spokeswoman Victoria Nuland on Monday would only characterize North Korea's statement on pulling out of the Korean War armistice as dangerous rhetoric, and not action. She called the statement concerning but said its impact was unclear.
Besides its vagueness, the U.N. says that North Korea cannot unilaterally dissolve the armistice. And neither South Korea nor the U.S. appears to have received any official communication stating that the cease-fire is no longer in effect.
Even with the armistice, the United States remains technically at war with the notoriously unpredictable North Koreans, whose opaque leadership has confounded successive American administrations. Some 28,500 U.S. troops are stationed in South Korea to deter potential aggression.