The US Secretary of State, Condoleezza Rice, has had talks in Tokyo with Japanese Foreign Minister Taro Aso about United Nations sanctions against North Korea.
The talks follow North Korea's first nuclear weapons test earlier this month.
Ms Rice said the two governments had pledged to work together for a swift implementation of the sanctions.
Ms Rice reassured Japan that the US would stand by a commitment to protect its Asian ally.
Her arrival in Tokyo coincided with a warning from intelligence experts that satellites had seen an increase in activity at North Korea's suspected test site, suggesting a second detonation was imminent.
Ms Rice is due to arrive in Seoul tomorrow as part of an Asian tour to rally support for the tough enforcement of UN sanctions imposed on Pyongyang last Saturday.
South Korea has so far expressed reluctance to take part because of China's opposition to the inspection measure.