South Africa told the United Nations in a confidential report that it seized arms traveling from North Korea by way of China, marking at least the third time a government interdicted North Korean weapons shipments since the U.N. last summer adopted harsher sanctions against Pyongyang.

The incident is likely to focus attention on China's role in enforcing sanctions against its ally and neighbor, with which it shares a long common border. China is by far North Korea's largest trading partner and its most important supplier of...