Sanctions alone will not change North Korea's behavior after this week's nuclear test and China needs encouragement -- not threats -- to alter its dealings with its renegade neighbor, a US expert said Thursday.

China's role in influencing North Korea is again in the spotlight following Pyongyang's fourth nuclear test. Beijing is the country's main ally and several figures including Republican presidential hopeful Donald Trump have demanded China do more.

Though North Korea already is subject to strict sanctions, the United Nations has promised a resolution to further strengthen these measures.

China, a permanent member of the Security Council, argues dialogue is the only way to moderate its behavior and remains a major exporter to North Korea.

Joseph DeThomas, a former ambassador and deputy assistant secretary of state for non-proliferation, told a Washington audience that beyond sanctions, the West needs to begin a longterm effort to encourage China to change how it deals with North Korea.

"We have to face the reality that sanctions alone will not leverage the North Korean policy in the absence of a fundamental change in Chinese policy," DeThomas said.

"For me, objective one in the diplomacy is how do we start creating change in Chinese policy to get them farther along."

Underlying everything is Beijing's desire to keep North Korea as a buffer between it and the prospect of US troops stationed on its border in a unified Korea.

But a nuclear-armed North Korea could result in a larger US buildup of anti-missile gear on China's doorstep.

"China is not going to like that reality," DeThomas said.