U.S. and South Korean officials have vowed to make North Korea pay a high price for its recent defiant nuclear test.

U.S. Assistant Secretary of State Antony Blinken is in South Korea on Wednesday on a diplomatic push to produce tougher sanctions that can force change in the North.

Key to those efforts is whether China, the North's last major ally and a veto-wielding U.N. Security Council member, will join in imposing any harsh punishment on the North.

Blinken met South Korea's Foreign Minister Yun Byung-se and said "We couldn't agree more on the need for a very clear and very strong international response."

Yun says it's time for the international community to stay united to make North Korea face the consequences for its bomb test.