By Yi Whan-woo

Smuggling on the North Korea-China border is thriving because border controls have eased following the summits between the countries' leaders in March and May, according to sources familiar with Pyongyang.

The sources said last week smugglers near the Yalu River had been trading goods banned under U.N. Security Council (UNSC) sanctions and that authorities were turning a blind eye.

"Smuggling is thriving and this is keeping the border very busy," a source said. "Many smugglers and firms whose job is to earn foreign currency are now focusing on providing products that their Chinese business partners want," a Radio Free Asia source said.

Another source said Chinese police and border guards "have definitely eased the crackdown since May" after smuggling had almost been stopped.

A third source said North Korean workers from trading companies and firms generating foreign currency were transporting banned products by ship and handing the products to their Chinese partners on the outskirts of China's Donggang and Dandong cities.

The sources said smuggling had never stopped, although it diminished after China joined the UNSC sanctions aimed at curbing North Korea's international trade over its nuclear program.

"To avoid inspection by Chinese border guards, those firms had to rent Chinese vessels, and the rental fees were determined by the ships' owners," a source said.

"Now, with Chinese authorities easing their restrictions, the rent is half of what it used to be."

The planned summit between North Korean leader Kim Jong-un and U.S. President Donald Trump in Singapore on June 12 fueled optimism for "even more smuggling" on the North Korea-China border, the sources said.

Kim visited China in March and May and met Chinese President Xi Jinping in what was seen as a bid to increase diplomatic leverage before meeting Trump.