U.S. Secretary of State Hillary Rodham Clinton said Friday in Tokyo that North Korea's sinking of a South Korean warship posed a threat to Japan and agreed with Foreign Minister Katsuya Okada to keep in close contact over the provocation.

During their joint news conference, Clinton clearly stated that Pyongyang was responsible for the sinking of the corvette Cheonan and added "the United States strongly condemns this action of aggression."

"The United States and Japan continue to work side by side to meet the challenge posed by North Korea," Clinton said. "We agree that North Korea must stop its provocative behavior, halt its policy of threats and belligerence toward its neighbors and take irreversible steps to fulfill its denuclearization commitments and comply with international law."