North Korean nuclear and rocket tests are drawing quick responses from the U.S. that will upset a supposed partner against Pyongyang's weapons development -- China.

New efforts to toughen missile defense in South Korea and sanctions legislation moving swiftly through Congress could both hurt Chinese interests. China is concerned the missile defense system could be used against it, and the U.S. sanctions could hit Chinese companies that trade with North Korea.

The U.S. and China both oppose North Korea's development of nuclear weapons. But a Jan. 6 atomic explosion and Sunday's rocket launch, condemned as a test of ballistic missile technology, have exposed stark differences between Washington and Beijing on how to deal with North Korean provocations.

Beijing is reluctant to impose stiff economic sanctions, fearing a North Korean collapse.