Speaking at a Senate confirmation hearing, Mrs Clinton also said that president-elect Barack Obama's administration would favor the current six-party nuclear disarmament negotiations and further efforts to bolster US-Asian alliances.
'We have got to end North Korea as a proliferator... So we will embark upon a very aggressive effort to try to determine the best way forward to achieve our objectives with them,' Mrs Clinton said during a question-and-answer session.
She recalled previously raised suspicions that North Korea transferred nuclear weapons technology to Syria and that it had sent nuclear know-how to Libya, which scrapped its nuclear program in a deal with Washington and London.
Mrs Clinton said she and Obama believe the current six-country negotiating basis involving the two Koreas, the United States, China, Russia and Japan have 'merit' after discussing it with outgoing Secretary of State Condoleezza Rice.
'But it also provides an opportunity, as Secretary Rice has testified before this committee, for bilateral contact as well between North Korea and the United States,' Mrs Clinton said without elaborating. 'Again this is under review.'
'Our goal is to end the North Korean nuclear program - both the plutonium reprocessing program and the highly enriched uranium program, which there is reason to believe exists, although never quite verified,' Mrs Clinton said.
'It is our strong belief that the six-party talks... is a vehicle for us to exert pressure on North Korea in a way that is more likely to alter their behavior,' the 61-year-old senator from New York told the Senate Foreign Relations Committee.
'I have no illusions about that. I think it takes tough, reality-based diplomacy to determine what is doable,' she said.
Negotiations are deadlocked over differences over how to determine whether North Korea is telling the truth about scrapping its nuclear programs.
In a sign of tension, North Korea vowed Tuesday not to give up its nuclear weapons until the United States drops its 'hostile' policy and establishes diplomatic relations.
The statement from a foreign ministry spokesman in the communist state reaffirmed current policy but comes just one week before the Obama administration assumes office in Washington.
The North in 2007 signed a six-nation disarmament pact which calls for the scrapping of its nuclear weapons in return for aid, normalized relations with the United States and Japan and a formal peace pact on the Korean peninsula.
It is disabling its nuclear plants under the latest phase of the pact but has not started negotiations on the final phase, which would involve the surrender of weapons and normalised relations.
The North frequently demands verification that US nuclear weapons have been withdrawn from South Korea. The US said this was done in the early 1990s.
In her opening remarks to the committee, she said Washington 'will continue to work to prevent proliferation in North Korea and Iran, to secure loose nuclear weapons and materials, and to shut down the market for selling them.' But such efforts required 'reaching out to both friends and adversaries to bolster old alliances and to forge new ones,' Clinton said.
'That means strengthening the alliances that have stood the test of time, especially with our Nato partners and our allies in Asia,' she said.
'Our alliance with Japan is a cornerstone of American policy in Asia,' she said.
'We also have crucial economic and security partnerships with South Korea, Australia, and other friends in Asean (Association of Southeast Asian Nations,' she said.
'We will build on our economic and political partnership with India,' she added. -- AFP