North Korea poses a mounting threat to the United States due to its pursuit of long-range missiles and nuclear weapons, the Pentagon said Tuesday in its latest strategy document.

Describing the regime in Pyongyang as "closed and authoritarian," the Defense Department said the US military would maintain a major presence in the region and keep up investments in missile defense.

The North represents "a significant threat to peace and stability on the Korean Peninsula and in Northeast Asia and is a growing, direct threat to the United States," said the Quadrennial Defense Review, an update of the military's global strategic outlook.

US forces would continue to collaborate closely with South Korea's military "to deter and defend against North Korean provocations," it said.

The release of the Pentagon's strategic review came as North Korea flexed its military might three times over the past week, firing short-range Scud missiles and rockets into the sea. The test launches were timed to coincide with joint US-South Korean drills that Pyongyang opposes.

The strategy document said the United States will seek to stay ahead of the threat of ballistic missile arsenals in Iran and North Korea, noting plans to bolster the number of ground-based interceptors on US soil from 30 to 44 while investing in better sensors.

The US administration also is deploying a second powerful surveillance radar in Japan to provide early warning of any missile launched by North Korea, it said.

North Korea has pressed ahead with its missile program but experts have voiced skepticism over its claims to have a working inter-continental ballistic missile.

To promote "stability" in the region, US forces will keep up "a robust footprint in Northeast Asia while enhancing our presence in Oceania and Southeast Asia," the review said.

Although Washington's much-touted strategic "rebalance" to the Asia-Pacific region has been criticized as more hype than substance, senior Pentagon officials insisted the review and a new budget proposal released Tuesday showed a commitment to the shift.

US officials cited ship building plans, deployments of marines to Australia and an expansion of joint military training and drills.

"We will continue our contributions to the US rebalance to the Asia-Pacific region, seeking to preserve peace and stability in a region that is increasingly central to US political, economic, and security interests," the review said.

At the same time, the US military would retain an "enduring" presence in the Middle East and the Gulf, where some 35,000 troops are stationed, while also keeping up ties to "stalwart" allies in Europe.

The document was drafted before the current crisis erupted in Ukraine, with pro-Russian forces taking de facto control over the Crimean peninsula.