China has reportedly tightened customs inspections of cargo trucks traveling between North Korea and China since the renegade country launched a long-range rocket in December last year.

"Since December, Chinese customs offices have inspected cargoes by the book at North Korea's major trading points, including Dandong," a North Korean source in Beijing said Wednesday. "The quantities being carried into the North have shrunk because checks are taking much longer."

The North relies on China for more than 90 percent of daily necessities since trade with South Korea was suspended in the wake of its sinking of the South Korean Navy corvette Cheonan and shelling of Yeonpyeong Island in 2010.

Beijing has threatened to cut aid to Pyongyang if it goes ahead with a nuclear test, China's official Global Times said recently. Pundits see the tightening of customs inspections as a warning message against provocations.

According to a source, Chinese customs officials until December turned a blind eye to North Korean traders carrying some food without customs clearance, but now they check that all cargo is declared.