SEOUL, South Korea — A senior U.S. diplomat stepped up pressure on China today to play a leading role in punishing North Korea for its recent nuclear test that raised worries about advancements in its bomb program.

U.S. Deputy Secretary of State Antony Blinken was in South Korea on a diplomatic push for tougher sanctions and punishment 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 such moves.

“We believe that China has a special role to play given the special relationship that it has with North Korea,” Blinken told reporters after meeting with South Korean officials.

He said Beijing has “more influence and more leverage” over Pyongyang than any other country because most its trade goes from, to or through China. “We are looking to China to show leadership on the issue,” Blinken said.

He flies to Beijing later today for talks on North Korea.

During 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 stand united to make North Korea face the consequences for its bomb test. “This is North Korea versus international community,” he said.

China is expected to join in some U.N. sanctions, but won’t likely go as far as to take steps that might lead to the collapse of the North’s authoritarian government. China fears the onslaught of a wave of refugees and violence surging across the border, analysts say.

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.