Russia is estimated to have supplied North Korea with more than a million barrels of oil since March this year, according to satellite imagery analysis from the Open Source Centre, a non-profit research group based in the UK.

The oil is payment for the weapons and troops Pyongyang has sent Moscow to fuel its war in Ukraine, leading experts and UK Foreign Secretary, David Lammy, have told the BBC.

These transfers violate UN sanctions, which ban countries from selling oil to North Korea, except in small quantities, in an attempt to stifle its economy to prevent it from further developing nuclear weapons.

The satellite images, shared exclusively with the BBC, show more than a dozen different North Korean oil tankers arriving at an oil terminal in Russia’s Far East a total of 43 times over the past eight months.

Further pictures, taken of the ships at sea, appear to show the tankers arriving empty, and leaving almost full.

North Korea is the only country in the world not allowed to buy oil on the open market. The number of barrels of refined petroleum it can receive is capped by the United Nations at 500,000 annually, well below the amount it needs.

Russia’s foreign ministry did not respond to our request for comment.

The first oil transfer documented by the Open Source Centre in a new report, was on 7 March 2024, seven months after it first emerged Pyongyang was sending Moscow weapons.

The shipments have continued as thousands of North Korean troops are reported to have been sent to Russia to fight, with the last one recorded on 5 November.

“While Kim Jong Un is providing Vladimir Putin with a lifeline to continue his war, Russia is quietly providing North Korea with a lifeline of its own,” says Joe Byrne from the Open Source Centre.

“This steady flow of oil gives North Korea a level of stability it hasn’t had since these sanctions were introduced.”

Four former members of a UN panel responsible for tracking the sanctions on North Korea have told the BBC the transfers are a consequence of increasing ties between Moscow and Pyongyang.

“These transfers are fuelling Putin’s war machine – this is oil for missiles, oil for artillery and now oil for soldiers,” says Hugh Griffiths, who led the panel from 2014 to 2019.

UK Foreign Secretary David Lammy has told the BBC in a statement: “To keep fighting in Ukraine, Russia has become increasingly reliant on North Korea for troops and weapons in exchange for oil."

He added that this was “having a direct impact on security in the Korean peninsula, Europe and Indo-Pacific".

While most people in North Korea rely on coal for their daily lives, oil is essential for running the country’s military. Diesel and petrol are used to transport missile launchers and troops around the country, run munitions factories and fuel the cars of Pyongyang’s elite.

The 500,000 barrels North Korea is allowed to receive fall far short of the nine million it consumes – meaning that since the cap was introduced in 2017, the country has been forced to buy oil illicitly from criminal networks to make up this deficit.

This involves transferring the oil between ships out at sea – a risky, expensive and time-consuming business, according to Dr Go Myong-hyun, a senior research fellow at South Korea’s Institute for National Security Strategy, which is linked to the country’s spy agency.

“Now Kim Jong Un is getting oil directly, it’s likely better quality, and chances are he’s getting it for free, as quid pro quo for supplying munitions. What could be better than that?"

“A million barrels is nothing for a large oil producer like Russia to release, but it is a substantial amount for North Korea to receive,” Dr Go adds.

In all 43 of the journeys tracked by the Open Source Centre using satellite images, the North Korean-flagged tankers arrived at Russia’s Vostochny Port with their trackers switched off, concealing their movements.

The images show they then made their way back to one of four ports on North Korea’s east and west coast.

“The vessels appear silently, almost every week,” says Joe Byrne, the researcher from the Open Source Centre. “Since March there’s been a fairly constant flow.”

The team, which has been tracking these tankers since the oil sanctions were first introduced, used their knowledge of each ship’s capacity to calculate how many oil barrels they could carry.

Then they studied images of the ships entering and leaving Vostochny and, in most instances, could see how low they sat in the water and, therefore, how full they were.

The tankers, they assess, were loaded to 90% of their capacity.

“We can see from some of the images that if the ships were any fuller they would sink,” Mr Byrne says.

Based on this, they calculate that, since March, Russia has given North Korea more than a million barrels of oil - more than double the annual cap, and around ten times the amount Moscow officially gave Pyongyang in 2023.

This follows an assessment by the US government in May that Moscow had already supplied more than 500,000 barrels’ worth of oil.

Cloud cover means the researchers cannot get a clear image of the port every day.

“The whole of August was cloudy, so we weren’t able to document a single trip,” Mr Byrne says, leading his team to believe that one million barrels is a “baseline” figure.

Not only do these oil deliveries breach UN sanctions on North Korea, that Russia, as a permanent member of the UN Security Council, signed off on – but also, more than half of the journeys tracked by the Open Source Centre were made by vessels that have been individually sanctioned by the UN.

This means they should have been impounded upon entering Russian waters.

But in March 2024, three weeks after the first oil transfer was documented, Russia disbanded the UN panel responsible for monitoring sanctions violations, by using its veto at the UN Security Council.

Ashley Hess, who was working on the panel up until its collapse, says they saw evidence the transfers had started.

“We were tracking some of the ships and companies involved, but our work was stopped, possibly after they had already breached the 500,000-barrel cap”.

Eric Penton-Voak, who led the group from 2021-2023, says the Russian members on the panel tried to censor its work.

“Now the panel is gone, they can simply ignore the rules,” he adds. “The fact that Russia is now encouraging these ships to visit its ports and load up with oil shows a new level of contempt for these sanctions.”

But Mr Penton-Voak, who is on the board of the Open Source Centre, thinks the problem runs much deeper.

“You now have these autocratic regimes increasingly working together to help one another achieve whatever it is they want, and ignoring the wishes of the international community.”

This is an “increasingly dangerous” playbook, he argues.

“The last thing you want is a North Korean tactical nuclear weapon turning up in Iran, for instance.”

As Kim Jong Un steps up his support for Vladimir Putin’s war, concern is growing over what else he will receive in return.

The US and South Korea estimate Pyongyang has now sent Moscow 16,000 shipping containers filled with artillery shells and rockets, while remnants of exploded North Korean ballistic missiles have been recovered on the battlefield in Ukraine.

More recently, Putin and Kim signed a defence pact, leading to thousands of North Korean troops being sent to Russia’s Kursk region, where intelligence reports indicate they are now engaged in battle.

The South Korean government has told the BBC it would “sternly respond to the violation of the UN Security Council resolutions by Russia and North Korea”.

Its biggest worry is that Moscow will provide Pyongyang with technology to improve its spy satellites and ballistic missiles.

Last month, Seoul’s defence minister, Kim Yong-hyun, stated there was a “high chance” North Korea was asking for such help.

“If you’re sending your people to die in a foreign war, a million barrels of oil is just not sufficient reward,” Dr Go says.

Andrei Lankov, an expert in North Korea-Russia relations at Seoul’s Kookmin University, agrees.

“I used to think it was not in Russia’s interest to share military technology, but perhaps its calculus has changed. The Russians need these troops, and this gives the North Koreans more leverage.”

Additional reporting by Josh Cheetham in London and Jake Kwon in Seoul