The U.S. government says it expressed concerns to China over reports regarding China’s alleged cooperation with North Korea on its missile program.

State Department spokeswoman Victoria Nuland told reporters on Wednesday that the U.S. has relayed its concerns to China but given the intelligence issues involved, what she can say is relatively limited.

Nuland added that the U.S will keep working with China and others in the international community to enforce UN sanctions on North Korea’s ballistic and nuclear missile programs.

Earlier, Japan’s Asahi Shimbun daily quoted Japanese government sources as saying that a Chinese company exported four vehicles to North Korea last August capable of transporting and launching ballistic missiles.

The daily also said that South Korea, the U.S. and Japan have not pressured China on the issue as the three countries need Beijing to pressure the North from conducting a third nuclear test.