Ukrainian President Volodymyr Zelenskyy said in a speech to parliament on Wednesday that North Korea was a de facto participant in the war in Ukraine, siding with Russia. He said that Ukrainian intelligence had found that Pyongyang was transferring not only weapons, but also soldiers to Moscow.

The deepening military ties between Russia and North Korea have earned condemnation from the United States, South Korea and Japan. The three countries on Wednesday announced a new team to monitor arms sanctions on North Korea.

So just how much is North Korea helping Russia, what’s the depth of their military cooperation, and to what extent does Moscow need Pyongyang’s help?

Is North Korea sending soldiers to Russia?

According to Ukraine and South Korea, yes.

On October 8, Seoul’s defence minister Kim Yong-hyun told South Korean politicians that it was “highly likely” that North Korean officers were killed in a Ukrainian strike near Donetsk on October 3.

And on Friday, October 18, South Korea’s National Intelligence Service said that Russian naval ships had transferred 1,500 North Korean soldiers to Russia’s Pacific port city of Vladivostok between October 8 and October 13.

However, Russia has dismissed the allegation that North Korean personnel are in Russia.

“This seems like yet another fake news story,” Kremlin spokesman Dmitry Peskov told reporters earlier this month.

While Ukraine and South Korea have not made public any evidence to back their claims, experts say that North Korean military presence in Ukraine is plausible.

“We cannot rule out the possibility,” Edward Howell, a lecturer in international relations at the University of Oxford, told Al Jazeera. “We know that Russia needs manpower.”

Howell added that even if North Korea is not sending foot soldiers, “we cannot discount North Korea sending military engineers, as well as personnel to assist in monitoring and supervising the usage of North Korean weapons – which may be numerous in quantity, but of variable quality – in Ukraine”.

Howell’s research focuses on the politics and international relations of North Korea, the Korean Peninsula and East Asia.

Zelenskyy earlier accused North Korea in a video address on Sunday of sending military personnel to fight for Russia against Ukraine.

In his Sunday video address, Zelenskyy said: “This is no longer just about transferring weapons. It is actually about transferring people from North Korea to the occupying military forces.”

“We see an increasing alliance between Russia and regimes like North Korea,” he warned.

Zelenskyy urged allies to step up their response to Russia, particularly in terms of lifting restrictions on Ukraine using long-range missiles to strike deep inside Russian territory.

“When we talk about giving Ukraine greater long-range capabilities and more decisive supplies for our forces, it’s not just a list of military equipment. It’s about increasing the pressure on the aggressor – pressure that will be stronger than what Russia can handle. And it’s about preventing an even larger war,” he said.

The United States has expressed concern over reports of a North Korean military presence in Ukraine – but has not independently made the accusation against Pyongyang itself.

General Charles Flynn, the US Army’s Asia Pacific commander, told an event in Washington that North Korean personnel being involved in the conflict would allow Pyongyang to get real-time feedback on its weapons for the first time.

“That kind of feedback from a real battlefield to North Korea to be able to make adjustments to their weapons, their ammunition, their capabilities, and even their people – to me, is very concerning,” he said, speaking at the Center for a New American Security on Tuesday.

What is the defence pact between North Korea and Russia?

North Korean leader Kim Jong Un, who rarely makes foreign trips, visited Russia in September 2023 and invited Russian President Vladimir Putin to visit North Korea.

In June, Putin made his first state visit to North Korea in 24 years, and Russia and North Korea signed a mutual defence pact. While the exact text of this pact was not released, the pact includes a mutual assistance clause calling on the two countries to provide military assistance should one of them be attacked.

On June 23, the US and regional allies South Korea and Japan released a joint statement published on the US State Department website, expressing “grave concern” over the pact.

Since then, Ukrainian troops carried out an incursion in Russian’s Kursk on August 6, in an act that could – in Russia’s reading – potentially constitute an attack, thereby triggering the mutual assistance clause in the agreement with North Korea.

On Tuesday, tensions spiked in the Korean Peninsula when North Korea blew up sections of roads near the South Korean border.

Also on Tuesday, Kremlin spokesman Dmitry Peskov was asked by reporters whether the mutual assistance clause means Russia and North Korea could be pulled into the Ukraine war or the Korean Peninsula conflict respectively. Peskov did not answer the question, merely saying that the treaty’s language was “quite unambiguous” and did not need to be clarified.

He told reporters that the pact “implies truly strategic deep cooperation in all areas, including security”.

Again, the US, Ukraine and South Korea say so while the Kremlin and Pyongyang deny it.

On October 9, the Ukrainian army said it struck a Russian weapons arsenal, which included weapons sent to Russia by North Korea. The army added that the drone attack on the Bryansk border region was aimed at creating logistical difficulties for Russia and limiting its offensive capabilities.

In the June 23 joint statement, the US, South Korea and Japan said that they condemn the deepening military cooperation between North Korea and Russia, including “continued arms transfers from the DPRK to Russia that prolong the suffering of the Ukrainian people”.

On February 27, then-South Korean defence minister Shin Won-sik told reporters that North Korea sent about 6,700 containers carrying millions of munitions to Russia since September 2023 in exchange for food and raw materials for weapons manufacturing.

In January, White House National Security Council spokesman John Kirby said that intelligence found that Russia used at least one weapon provided by North Korea into Ukraine on December 30, 2023. The weapon landed in an open field in the Zaporizhia region, Kirby said. He said that more weapons provided by North Korea were used on January 2.

In April, Reuters news agency reported that United Nations sanctions monitors told the UN Security Council that debris from a missile that landed in the Ukrainian city of Kharkiv on January 2 was identified to be from a North Korean Hwasong-11 series ballistic missile. This is a violation of the arms embargo on North Korea.

North Korea has been under UN sanctions for its ballistic missile and nuclear programmes since 2006, and those measures have been strengthened over the years.

In March, Russia vetoed the UN’s renewal of a panel of UN experts monitoring North Korea’s compliance with the sanctions. While the sanctions will remain in place, the monitoring force will not.

Why is the military relationship between Moscow and Pyongyang getting deeper?

International relations lecturer Howell told Al Jazeera that the relationship stemmed from a “largely transactional” need.

After Russia launched its full-scale invasion of Ukraine in February 2022, it was facing global isolation, and “North Korea was able to provide the goods that Putin wanted, in exchange for getting what it needed in return”.

Howell said that with the signing of the defence pact, a “cash-for-weapons” relationship was established. “North Korea provided artillery, which soon escalated into providing ballistic missiles, and in return, Russia provided food, cash, and, crucially, assistance in military technology”. The advanced military technology is crucial for North Korea, Howell explained, “since Kim Jong Un’s ultimate goal remains for North Korea to be recognised as a de facto nuclear state.”

Besides material weapons, Pyongyang gains Moscow’s “unwavering support” in the UNSC, Howell said. “Pyongyang can thus escape scot-free if it chooses to bolster its nuclear and missile programme through testing and launches, which, as we know, is what North Korea intends to do.”