The northeastern Chinese border city of Dandong has tightened checks on goods being shipped to and from North Korea since the North's fourth nuclear test, although the shipment of goods between the allies appears to be business as usual, two sources with knowledge of the matter said Tuesday. 

China had previously stepped up customs checks on goods being traded with North Korea in Dandong, where about 70 percent of bilateral trade between the allies is being conducted, after the North's third nuclear test in early 2013. 

But few analysts believe that China would take punitive steps to punish North Korea for conducting the latest nuclear test on Jan. 6. 

A source with knowledge of trade with North Korea said Chinese customs authorities in Dandong "have been monitoring shipments in a stricter manner than usual since North Korea's test of a hydrogen bomb." 

"If there are goods without proper documents, the Chinese authorities relentlessly filter them out," the source said. 

North Korea announced it had successfully detonated a hydrogen bomb on Jan. 6, though many outside experts expressed doubts about the North's claim. Whatever the type of nuclear test, it marked Pyongyang's fourth nuclear test since 2006. 

Another source with knowledge of trade with North Korea said, "Almost all negotiations with Chinese businessmen have been suspended since the nuclear test." 

Outwardly, the flow of shipments between Dandong and North Korea appears to be unaffected by the North's nuclear test, the two sources said. 

South Korea and the United States have called on China, which keeps North Korea's economy afloat as a key supplier of food and oil, to back tougher U.N. sanctions against the North for its latest nuclear test. 

China's reaction to such calls has been lukewarm. Many analysts believe that China's Communist Party leadership won't exert leverage on North Korea because a sudden collapse of the North's regime could threaten China's own security interests.

Two-way trade between North Korea and China fell 14.7 percent on year to $5.43 billion in 2015, according to data by South Korea's Korea Trade-Investment Promotion Agency. (Yonhap)