Successful rocket tests have propelled North Korea ahead in its two-decade, virtually one-sided arms race with Japan, leaving the government unsure it could fend off a missile strike without U.S. help, according to defense sources.

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 Self-Defense Forces 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.