Russian smugglers are illicitly supplying North Korea with contraband energy and goods, evading sanctions and ignoring concerns about the isolated Asian country’s nuclear tests, the Washington Post reported Monday.

“As the Chinese cut off oil and gas, we’re seeing [North Korea] turn to Russia,” the newspaper quoted an unnamed senior U.S. official with knowledge of alleged Russian smuggling into North Korea as saying on Monday.

Officials and experts reportedly noticed a sharp rise in supplies of Russian diesel and other fuels to North Korea starting in spring 2017.

The newspaper said a “dedicated ferry line” was launched between Russia’s Far East and North Korea to accommodate heavy traffic. The spike coincided with new UN sanctions and a ban on Chinese government fuel shipments in May, according to the Post.