Speaking at a news conference with Japanese Foreign Minister Hirofumi Nakasone, Clinton said a possible North Korean missile launch "would be very unhelpful in moving our relationship forward," CNN reported.
North Korean officials have denied plans to conduct a long-range missile test-firing, saying they were launching a satellite.
During her first diplomatic trip, Clinton also extended an invitation from U.S. President Barack Obama to Japanese Prime Minister Taro Aso to meet him in Washington, The New York Times said. Aso would be the first foreign leader received at the White House in the new administration.
Clinton said she also met with families of people abducted by North Korea "to express my personal sympathy and our concern for what happened."
During her news conference with Nakasone, Clinton said Washington wouldn't ease its pressure on North Korea to give up its nuclear weapons program but she restated Obama's pledge to "reach out a hand to those with which we have differences, so long as they unclench their fists," the Times reported.
She also committed to continuing the multiparty talks on denuclearizing the Korean Peninsula that include China, Japan, Russia and the two Koreas.
Nakasone said he wasn't concerned that "the U.S. policy toward North Korea is going to change in any significant way."