Six ships that have been traced back to China were found by U.S. intelligence to have been assisting North Korea, violating sanctions placed on the rogue regime by the United Nations, according to a report.

U.S. officials identified and gathered information about the movement of the cargo ships, which are owned or managed by Chinese companies, using satellite imagery and other intelligence means. The evidence was presented to a United Nations sanctions committee, the Wall Street Journal reported Thursday.

After the U.S. presented the information to the U.N., it asked the body to formally declare a total of 10 cargo ships as sanctions violators. China pushed back on the request, but it did permit the U.N. to blacklist four ships, excluding the six vessels tracked by the U.S., that don’t have connections to Chinese companies.

According to declassified intelligence reports, photos and maps given to the U.N. and reviewed by the Journal, the ships picked up illegal cargo, primarily coal, in North Korea and either transported it to Russia and Vietnam, or transferred it to other ships while at sea.

Some of the vessels disguised their locations by turning off their Automatic Identification Systems, which show a ship’s location.

The Journal used corporate records and shipping databases to determine the six ships are owned by Chinese companies registered in Hong Kong. Chinese nationals invest in the companies, and they have used addresses in China.

The Chinese government has investigated at least four of the six cargo ships and officials have questioned several of their owners and managers. At least one of the vessel’s managers has been arrested, the Wall Street Journal reported.

In December, the U.N. Security Council adopted a ban of 90 percent of refined petroleum exports to North Korea.

The United States has voted to impose sanctions on North Korea following its nuclear and missile tests.

President Trump and his administration have also called for China to assist in pressuring North Korea to end its nuclear program.

In an interview with Reuters published Wednesday, the president said China has been assisting the U.S. with North Korea, but he accused Russia of countering that help.

“Russia is not helping us at all with North Korea,” Trump said. “What China is helping us with, Russia is denting. In other words, Russia is making up for some of what China is doing.”

The president said China could be doing more to pressure North Korea, but applauded Beijing’s actions to limit oil and coal to North Korea, according to Reuters.