Japanese officials say China has violated a U.N. embargo by supplying North Korea with vehicles capable of transporting and launching ballistic missiles.

Local media Wednesday quoted government sources as saying that a Chinese company sent four giant, 16-wheel missile launch vehicles to North Korea last August.

The Asahi Shimbun, which first reported the story, said the U.S. has not publicly criticized China over the matter because it does not want to embarrass Beijing and because it needs China's support to help stifle Pyongyang's nuclear ambitions.

China has denied breaking the U.N. resolutions, which prohibit supplying Pyongyang's weapons program with arms, training and other assistance.

In April, U.S. Defense Secretary Leon Panetta acknowledged that China has provided “some help” to North Korea with its ballistic missile program through trade and technology exchanges.

His comments came after Western defense experts spotted what appeared to be Chinese-made missile launch vehicles prominently displayed at a military parade in North Korea, which does not have the technology to make such vehicles.

The Asahi Shimbun said in its report Wednesday the vehicles exported from China were likely the same ones in the military parade.

The report said the vehicles were transported on a Cambodia-registered ship, which was tracked U.S., Japanese and South Korean spy satellites as it arrived in North Korea on August 4. Japanese coast guard officials later inspected the vehicle and found a document detailing the export of the vehicles.

After defense experts spotted the missile launch vehicles in April, China's foreign ministry vigorously denied any wrongdoing on Beijing's part, saying China had not violated the resolution.

Beijing has been uncharacteristically critical of its ally North Korea following Pyongyang's failed satellite launch in April, which Western nations said was a disguised long-range missile test – also banned under U.N. sanctions.