SEOUL, South Korea >> U.S. and South Korean officials vowed Wednesday to make North Korea pay a high price for its defiant nuclear test earlier this month that caused worries about advancement in the North’s bomb program.

U.S. Assistant Secretary of State Antony Blinken was in South Korea on a diplomatic push for 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.

“We couldn’t agree more on the need for a very clear and very strong international response,” Blinken said at the start of a meeting with South Korean Foreign Minister Yun Byung-se.

Blinken said Seoul and Washington are working closely in New York with the United Nations Security Council

Yun said it is time for the international community to stay united to make North Korea face the consequences for its bomb test. “This is North Korea versus international community,” he said.

North Korea says it conducted a hydrogen bomb test on Jan. 6. Many governments and experts remain highly skeptical about the North’s claim, but whatever device North Korea detonated will likely push the country a step closer toward its goal of manufacturing a miniaturized warhead to place on a missile that can threaten the U.S. mainland.

After the bomb test, the rival Koreas resumed psychological warfare with Seoul blasting anti-Pyongyang broadcasts from border loudspeakers, while Pyongyang does the same and also floats propaganda leaflets over the border by balloon, according to South Korean officials.