Such transactions could have helped provide North Korea with an economic lifeline and eased the isolation of the secretive communist state, whose leader, Kim Jong Un, is due to meet U.S. President Donald Trump in Vietnam this week.

Primportbunker, the owner of the vessel the crew members said made the transfers, did not respond to requests for comment by telephone. No one answered the door when Reuters visited the building where Primportbunker has its headquarters in the port city of Vladivostok on Russia's Pacific coast.

On the four voyages between Oct. 13, 2017, and May 7, 2018, the Tantal tanker gave its destination as the Chinese port of Ningbo when it set sail, according to port documents seen by Reuters and tracking data from financial data company Refinitiv.

It then met up in international waters with a North Korean vessel to which it transferred its cargo of fuel, the two crew members who witnessed the transfers said.

The two crew said the fuel transfers took place when the Tantal's transponder, which allows the vessel to be tracked at sea, was not operating. Shipping industry experts said this indicates the transponder was deliberately turned off or the Tantal had entered a zone not covered by ship-tracking radar.

On each occasion, the transponder started operating again when the Tantal was close to port in Russia, the two crew said.

They declined to give their names, citing fear of reprisals.

"We got officially registered for Ningbo and went to the 12-mile zone (marking the limits of Russian territorial waters)," one of the crew said, describing four journeys in which he was involved.

"We worked at night there with the North Korean tanker Chon Moyng-1," he said.

Such transactions violate the international sanctions imposed on North Korea over its nuclear and missiles programme, which include a United Nations ban on nearly 90 percent of refined petroleum exports to Pyongyang.

Washington has accused Russia of "cheating" on sanctions and said it has evidence of "consistent and wide-ranging Russian violations". In earlier denials that it has violated sanctions, Russia has said such accusations are not backed up by evidence.

Alexander Matsegora, Russia's ambassador to North Korea, was quoted by Interfax news agency as saying the allegations over the Tantal were "rumors apparently collected in certain kinds of establishments in the port."

He said any Russian fuel deliveries to North Korea were legal and mainly by rail.

REUTERS