U.S. State Department spokeswoman Victoria Nuland says the United States has raised concern to China over allegations that elements in the country supported North Korea's missile program.
Nuland said Washington brought up the issue "in recent weeks" but declined to add further details.
Her comments came after Japanese media reported on June 13 that a Chinese company shipped to North Korea four giant trucks capable of transporting and launching ballistic missiles last year, in breach of a UN Security Council resolution.
The reports said the vehicles were likely those on display at the huge military parade in April marking the centenary of the birth of North Korea's founder, Kim Il Sung.
China has called the Japanese reports inaccurate, and denied violating any UN restrictions.
A Japanese government spokesman declined to confirm the reports.
Based on reporting by AFP and Reuters