A top US military official on Wednesday called for better missile defense cooperation between Japan and South Korea, in the face of strained ties between America's two closest Asia allies and a belligerent North Korea.

"We're encouraging our allies and partners to acquire their own missile defenses and to strengthen regional missile defense cooperation that will result in better performance than individual countries acting alone," said James Winnefeld, vice-chairman of the Joints Chief of Staff.

"We will continue to emphasize the importance of developing regional ballistic missile defense systems," Winnefeld said during a speech at the Atlantic Council think tank.

"This is a very politically sensitive topic for several of our regional allies, but progress in this area would only increase our confidence in the face of persistent North Korean provocations," Winnefeld said.

"This is about ensuring we can deny the objectives of any insecure authoritarian state that believes acquisition of deliverable weapons of mass destruction is key to the preservation of its regime."

Winnefeld said that in terms of weapons capability Pyongyang poses the greatest threat, "followed by Iran."

His appeal comes with relations between Seoul and Tokyo at their lowest level in years, strained by Japan's 1910-45 colonial rule of Korea and a territorial dispute over islets in waters between the two countries.

Despite those regional tensions, Washington likely will "come to rely more" on its Asian allies "to resource the means for their defense," the general said -- especially "in a world of declining budgets."

North Korea's nuclear and ballistic missile program is a major security concern in the Pacific region and beyond.

Despite international isolation and extensive sanctions, Pyongyang appears to be readying a fourth nuclear test, observers have said.

While it's among the states most concerned about North Korea, resource-poor Japan has maintained friendly relations with oil-rich Iran through its years of ostracism, keeping up a diplomatic dialogue during Tehran's decades long confrontation with Washington.

The Wall Street Journal reported that Washington is weighing a plan to deploy an advanced missile-defense system in South Korea, one that could intercept short, medium and intermediate missiles.

The anti-missile system THAAD, short for Terminal High-Altitude Area Defense, is similar to one deployed by the United States to protect bases in its territory of Guam.

A Pentagon spokesman, Colonel Steven Warren, said Wednesday that while THAAD is "a very capable system... as of today, there's been no decision" by Seoul or Washington to deploy a battery in South Korea.

Winnefeld argued that a regional approach to missile defense could help spread the costs, noting that a single Thaad missile interceptor costs around $11 million compared to $3 million for a Scud, North Korea's preferred missile.

Meanwhile, Winnefeld said the United States will deploy an additional TPY-2 radar in Japan by the end of 2014 "to both improve our homeland and regional defense capabilities."

He added that the United States is also continuing to operate the Sea-based X-Band Radar (SBX) "as needed in the Pacific" and is planning to deploy a new, long-range radar for the Pacific region around 2020.