The U.S. Congressional Research Service says China has apparently not changed its fundamental policy toward North Korea despite the North conducting its second nuclear test last year.

The service said in a report that Beijing seems to have stuck to its existing policy toward Pyongyang because it considers the North's nuclear program not as a direct threat to China but a challenge to U.S. interests.

The report says China might be buying time intended for regional stability or has yet to acknowledge strong U.S. pressure on itself.

The congressional service also says China is intervening more in North Korea affairs rather than isolating the Stalinist country, an approach which can help Pyongyang return to dialogue but could also trigger side effects if the North retains its nuclear weapons.