The United States has expressed concern amid signs that North Korea could be preparing to test its longest-range missile.

State Department deputy spokesman Robert Wood told reporters Wednesday that a ballistic missile launch by North Korea would be "unhelpful and provocative."

Wood cited that U.N. resolution 1718 prohibits North Korea from engaging in any missile-related activities. Adopted in October 2006, the resolution imposed a series of economic and commercial sanctions on the North after Pyongyang’s claimed nuclear test on October ninth of the same year.

Earlier on Tuesday, a U.S. counter-proliferation official said that there were signs that the North Koreans are preparing for a Daepodong Two missile launch.