Some North Korean traders are offering cheap coal to Chinese buyers who are stockpiling it at ports inside the isolated country, hoping recent diplomatic moves lead to an easing of sanctions barring purchases of North Korean coal, three Chinese traders told Reuters.

Official data shows China has not imported any coal from North Korea since October last year, after the United Nations banned Pyongyang from exporting coal in September.

In 2016, China, Pyongyang’s main trading partner, bought 22.5 million tonnes of coal from North Korea worth almost $2 billion.

But the Chinese traders said offers of coal had surged after North Korean leader Kim Jong Un made a surprise visit to Beijing in March, and ahead of a planned meeting with US President Donald Trump.

“The day Kim Jong Un visited Beijing, I was approached by a North Korean trader asking if I wanted inventory at Nampo port,” one coal trader based in northern China told Reuters, referring to North Korea’s main west coast port.

The North Korean trader had a couple thousand tonnes of anthracite coal which he was willing to sell at around $30-$40 per tonne, the trader said, less than a quarter of the price of similar Chinese coal.

The two other Chinese traders confirmed that price range. All three requested anonymity because of the sensitivity of the situation.

Prices were even lower during the Lunar New Year period in February, hovering at less than $15 per tonne, said another of the traders, who is based in northeastern China.