A senior U.S. diplomat warned China on Thursday against selling nuclear and missile-related technology to North Korea, ratcheting up pressure on Beijing in the wake of Pyongyang's fourth nuclear test.

Thomas Countryman, assistant secretary of state for international security and nonproliferation, said the issue has been a subject of "constant dialogue and cooperation" between Washington and Beijing.

"When North Korea and Iran seek to purchase high technology materials or equipment for their nuclear and missile programs, the place that they like to shop is China," he said in a teleconference with reporters in the Asia-Pacific region. "It is a very high technology economy, and it produces a number of advanced goods, and it is important that China exercise the same degree of vigilance and control on strategic trade with Iran and North Korea as other countries do."

On Wednesday, U.S. Deputy Secretary of State Tony Blinken urged China to "show leadership" on the North Korean nuclear issue, saying Beijing has more leverage over Pyongyang than any other country.

He noted that nearly all of North Korea's trade goes to, from or through China.

The U.S., South Korea and other countries are trying to draw a strong sanctions resolution from the U.N. Security Council to punish North Korea for its nuclear test on Jan. 6.

China's role will be critical in the process as it is one of five veto-wielding permanent members of the council. However, it has so far shown little resolve to pressure its ally apparently out of concerns about its own national interests.

Blinken is currently in Beijing for talks with Chinese officials on the issue, while U.S. Secretary of State John Kerry is scheduled to visit the country later this month. (Yonhap)