Senior Japanese and U.S. diplomats agreed Monday to strengthen coordination in responding to North Korea's continued pursuit of nuclear and missile technology amid signs the reclusive state may conduct a fifth nuclear test.

"We need to continue to take effective measures against North Korea, always keeping in mind that the country may conduct further provocative acts," Vice Foreign Minister Akitaka Saiki told reporters after his talks with Deputy Secretary of State Antony Blinken in Tokyo.

Earlier Monday, South Korea said signs indicate North Korea is preparing to conduct its fifth nuclear test.

Saiki and Blinken met ahead of a trilateral meeting involving South Korean Vice Foreign Minister Lim Sung-nam in Seoul on Tuesday, in which they are expected to confirm defense and security cooperation.

Saiki said the trilateral meeting will cover ways to ensure that U.N. Security Council sanctions in response to North Korea's provocative actions are implemented effectively.

North Korea has continued to launch missiles, the latest being the failed launch on Friday of what was believed to be a Musudan medium-range ballistic missile, despite the imposition of U.N. sanctions following its fourth nuclear test in January and the launch of a rocket using banned ballistic missile technology in February.

Saiki and Blinken also exchanged views on the contentious plan to relocate U.S. Marine Corps Air Station Futenma within Okinawa Prefecture, as well as on maritime safety issues, Saiki said.

Before meeting with Saiki, Blinken met with Foreign Minister Fumio Kishida.

Japan and the United States are facing "the immediate challenge of the DPRK and its ongoing provocative actions with its nuclear and missile program," he told Kishida, referring to North Korea's official name, the Democratic People's Republic of Korea.