Dandong, China ? Customs officials examined trucks at the North Korean border Monday as China complied with new U.N. sanctions on Pyongyang for its nuclear test. But China’s U.N. ambassador indicated its inspectors would not board ships to search for suspicious equipment or material.

The United States began a new round of diplomacy in Asia to address divisions over how to impose the sanctions, with Secretary of State Condoleezza Rice to arrive in Japan on Wednesday before traveling to South Korea and China. The U.S. announced that air samples gathered last week contain radioactive materials confirming that North Korea conducted an underground nuclear explosion, as it claimed.

China’s support is key to whether the measures will have any impact on neighboring North Korea. Beijing’s mixed response on implementing the sanctions, approved Saturday by a unanimous U.N. Security Council including China, demonstrates the difficulties U.S. diplomats will encounter as they tour the region.

China and Russia contend that interdicting ships might needlessly provoke the North and at the very least discourage it from returning to talks on its nuclear program – though the U.S. and Britain say most inspections of ships would be done at ports rather than on the high seas. Australia announced it was banning the North’s ships from entering its ports, except in dire emergencies.

While China is angry over its communist ally’s behavior and is loath to appear out of step with other powers, it has been reluctant to support or implement tough measures. The leadership is concerned that tightening the squeeze on Pyongyang might trigger a collapse of the North Korean regime, sending refugees streaming across the border.

The sanctions ban trade with the North in major weapons and materials that could be used in its ballistic missile and weapons of mass destruction programs. They call for all countries to inspect cargo to and from North Korea to enforce the prohibition.

North Koreans stand near a stockpile of goods delivered to the waterfront of the North Korean city of Sinuiju, opposite the Chinese border city of Dandong. While China inspected trucks at the border on Monday, it refused to inspect cargo aboard ships.

In some border areas, the Chinese seem to have stepped up inspections, though elsewhere a police officer said nothing had changed and inspections were continuing as usual.

At a border-crossing post in the Chinese city of Dandong, about 30 Chinese trucks were seen being checked on Monday morning while 50 empty North Korean trucks waited in line to enter China to pick up cargo.

Customs officers opened the back of each truck and looked at its cargo as it rolled up, though they didn’t open individual boxes or bags. By contrast, reporters who visited the border post last week didn’t see inspectors open any trucks.

In the afternoon, the officers repeated the process as loaded North Korean trucks returned home. They climbed into the back of the vehicles, but observers couldn’t see whether they opened any containers.

Trading companies in Dandong, at the western end of the border, and in Tumen, near the eastern end, said the sanctions were not affecting shipments.

“Today, we just sent a batch of agricultural tools to North Korea by truck,” said Huang Kelin, manager of Wanshida Trading Co., a Dandong-based firm that has an office in Pyongyang.

At the Nanping crossing, in an eastern valley surrounded by mountains, inspectors were going through a standard regimen, looking at both cargo and passengers, a police officer said. “The inspections are routine and conducted by quarantine officials,” said the officer, Li Canhao.

Chinese goods reach the North by road and rail, while oil is delivered mainly via pipeline. It wasn’t immediately clear what China was doing to inspect rail shipments. A lone locomotive headed into North Korea on Monday afternoon, apparently to pull a cargo train back to China. The North also has a rail link to Russia in the east, though it wasn’t clear how that was being policed.