North Korea escalated its war rhetoric after it was censured by the U.N. Security Council for a February underground nuclear test, the country's third.
Japanese Ambassador to South Korea Koro Bessho, said there was a need to improve regional defensive ties given heightened security concerns in the region.
"North Korea has threatened a pre-emptive nuclear attack against South Korea and Japan," he was quoted by South Korea's Yonhap news agency as saying. "Against the backdrop of North Korea's threats, there is the need to forge a close coordination among Japan, South Korea and the United States."
The South Korean government said Wednesday it secured 36 Apache attack helicopters from Boeing under the terms of a $1.6 billion contract. Delivery is expected by 2018.
South Korean military drills with U.S. forces sparked a strong reaction from North Korea.
U.S. Marine Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, is scheduled Thursday to speak by video conference to his South Korean counterpart, army Gen. Jung Seung-jo, to discuss security on the Korean Peninsula.