The United States is urging China to help the U.N. Security Council work out effective countermeasures against North Korea's recent nuclear blast that threatens global peace, the top U.S. envoy in Seoul said Wednesday.

North Korea has claimed it successfully carried out a nuclear test on Feb. 12, its third since 2006, with a "miniaturized" bomb, prompting the U.S., South Korea and other regional powers to explore ways to further tighten existing sanctions against the North.

The U.N. Security Council is currently working on measures to tighten earlier sanctions imposed on North Korea for its banned missile and nuclear activity.

"We are working very closely with the South Korean government and of course working closely with the Chinese government as well to try to come up with best possible response to the test," U.S.

Ambassador Sung Kim said in a speech to a group of South Korean business leaders at a Seoul hotel.

Kim said China, as a permanent member of the Council, has a "very important role to play" and due responsibility for any action aimed at curbing the North's provocative behavior.

China expressed clear opposition to the North's nuclear test, but it remains quiet in the ongoing debate over how to punish its isolated ideological ally.

China, as a major political and economic patron of North Korea, is widely believed to have significant leverage over its impoverished neighbor. Still, it has been reluctant to support any tougher measures against the North.

The U.S. ambassador was also critical of calls among some South Koreans that the U.S. should re-deploy its tactical nuclear weapons to the Korean Peninsula or allow South Korea to develop its own nuclear capability as a countermeasure.

"I believe that would be a huge mistake," Kim said, referring to the South Korean calls.

Kim said the U.S. and South Korea instead need to maintain the strongest possible combined deterrent capability so that North Korea will not be tempted to launch an attack against South Korea.

He also said the U.S. will not hand over the planned transfer of its wartime operational command to South Korea if Seoul is not ready to assume the responsibility.

South Korea is scheduled to take over wartime operational control of its military from the U.S. in 2015.

"We will not transfer the OPCON if the South Korean side is not prepared," Kim said, using the acronym for the operational command control.

He said the transfer of the wartime operational command will take place only when Seoul and Washington decide that the South Korean military is ready to assume the responsibility.

The U.S. has held wartime command of South Korean troops since the beginning of the 1950-53 Korean War, though it handed over peacetime control of the South Korean military to Seoul in 1994.

The U.S. keeps some 28,500 troops in South Korea as a deterrent against North Korea, a legacy of the war that ended in a cease-fire, not a peace treaty.

(Yonhap News)