AKIPRESS.COM - Successful rocket tests have propelled North Korea ahead in a two-decade long arms race with Japan, leaving Tokyo unsure it could fend off a missile strike by the Pyongyang regime without U.S. help, military sources told Reuters.

Under young leader Kim Jong Un, North Korea has test fired 21 ballistic missiles since the start of the year, an unprecedented burst of activity that has rattled its neighbors and the international community.

"Their progress has been faster than anticipated," a senior Japanese military commander said. "There is a limit to what our current ballistic missile defense system can achieve," he added, asking not to be identified because he isn't authorized to speak to the media.

Planned upgrades to Japan's ballistic missile defense (BMD) are not due to begin until April at the earliest, while the deployment of new systems designed to destroy incoming warheads could take years to complete.

Constrained by production schedules and tight budgets that limit its ability to accelerate those plans, Japan may instead have to lean more heavily on its U.S. ally to guard against attacks, the sources said.

"Our only option for now may be to rely on the U.S. to stop them," said another source at Japan Self Defence Forces (SDF).