Top envoys from Japan and the United States on Sunday agreed to seek the "strongest possible measures" against North Korea, two days after the reclusive state conducted its fifth nuclear test in the face of global opposition.

"We will be working together very closely in the (U.N.) Security Council and beyond to come out with the strongest possible measures against North Korea's actions," Sung Kim, U.S. special representative for North Korea policy, told reporters following his meeting with Kenji Kanasugi, director-general of the Foreign Ministry's Asian and Oceanian Affairs Bureau in Tokyo.

Kim added that the two countries will also look at unilateral measures, bilateral measures and trilateral cooperation with South Korea in response to "provocative and unacceptable behavior by the North Koreans."

The U.S. special envoy on North Korea is visiting Japan and South Korea through Tuesday following the nuclear test and recent ballistic missile launches.

Kim did not elaborate on the measures, saying, "We don't have any specific measures to announce today but I can assure that both the United States and Japan are looking at full range of possibilities in terms of unilateral actions that can be implemented."

Following Friday's nuclear test, the U.N. Security Council convened an emergency meeting the same day, where they vowed to promptly work on "appropriate measures" in response to the isolated country's actions, including a reference to a possible sanctions resolution.

"There is a strong commitment in the Security Council that we must come up with strongest possible response as quickly as possible. We are just getting started with that process so I think it's too quick to predict the exact timeline at this juncture," Kim said.