Secretary of State Hillary Clinton says the United States is watching North Korea very closely to see if it ends "provocative language and actions".
Speaking in Japan, Ms Clinton promised that the US would offer North Korea aid if it completely eliminated its nuclear programme.
On Monday, Pyongyang announced that it was preparing to launch a long-range missile as part of a space programme.
Mrs Clinton is in the Japan on the first leg of an Asian tour.
She began her trip to Japan with a visit to the Meiji shrine and a meeting with US embassy staff.
She took part in a ceremony at the shrine, dedicated to the father of modern Japan Emperor Meiji, where the priest stressed balance and harmony.
"It's not only a good concept for religious shrines, it's a good concept for America's role in the world," Mrs Clinton said.
She had earlier described America's relations with Asia as indispensable.
"We need to be looking to create more balance, more harmony," she said.
Hillary Clinton: 'We must advance efforts towards a complete denuclearisation'
Her comments came after sabre rattling by North Korea amid speculation that Pyongyang is preparing to launch a long-range missile, which it says is part of its space programme.
Representatives of the US, South Korea, North Korea, China, Russia and Japan will be holding security talks in Moscow at the end of the week as part of efforts to re-start the Six Party Talks intended to bring an end to North Korea's nuclear programmes.
"The North Koreans have already agreed to dismantling. We expect them to fulfil the obligations that they entered into," Mrs Clinton told reporters en route to Japan.
Japanese stress
Mrs Clinton chose to visit Asia on her first trip as US secretary of state
On her arrival in Tokyo on Monday, Mrs Clinton stressed the "vitally important" alliance between Japan and the US - something welcomed in comments by Japanese Foreign Minister Hirofumi Nakasone.
Her visit comes at a time of intense economic and political stress for the Japanese government of Prime Minister Taro Aso.
His popularity has plunged to new lows, his finance minister has been forced to resign, and the economy has shrunk so much that a minister likened the situation to the end of World War II.
"These are hard times economically for the Japanese people, just as it is in many places around the world," Mrs Clinton said.
"I am absolutely confident we will navigate our way through these difficulties," she said.
She is scheduled to meet the leader of the main opposition Democratic Party of Japan, Ichiro Ozawa, to have tea with the Japanese empress and to meet the families of Japanese citizens who were abducted decades ago by North Korean agents.
Mrs Clinton will also sign an agreement to move about 8,000 Marines on the Japanese island of Okinawa to the US Pacific territory of Guam.
Asian partners
This is the first time Asia has been the initial trip for a new US secretary of state since the 1960s.
By making Japan her first stop, Mrs Clinton is seeking to reassure America's key ally in the region that its relationship with Washington is still strong and will remain so, says the BBC's state department correspondent.
Mrs Clinton will then visit Jakarta, followed by Seoul. But her final stop, Beijing, will be at the heart of the tour, our correspondent adds.