Kiev, Ukraine – Ukrainian intelligence believes North Korea has expanded its support for Russia’s war in Ukraine from supplying weapons to sending thousands of its own soldiers to fight for the Kremlin, a source tells NBC News.

The news was previously reported by various media.

“The first units are already being formed for deployment in the border areas and Russian areas,” the intelligence source said on Tuesday.

Several thousand soldiers were trained in Russia’s Far East, the source added.

“These units are also being equipped and prepared for dispatch,” possibly toward Kursk, a Russian region where Ukrainian forces launched an incursion in August.

The first group of North Korean soldiers is currently in Ulan-Ude, the capital of Russia’s Buryatia region, the source said.

On Sunday, Ukrainian President Volodymyr Zelensky said North Koreans were fighting with Russia and that there was “a growing alliance between Russia and regimes like North Korea.”

“This is no longer just about transferring weapons,” he said in his nightly video address. “It’s actually about transferring people from North Korea to the occupying forces.”

The Foreign Ministry said on Tuesday it was concerned about reports of North Korean soldiers fighting for Russia.

“If true, it would represent a significant increase in the relationship between these two countries, the relationship you’ve seen developing in recent months,” spokesman Matthew Miller said.

When asked about the accusation on Thursday, Kremlin spokesman Dmitry Peskov said it was a hoax.

It comes at a time when Russia is facing multiple setbacks not only in Ukraine, where it is grappling with casualties and depleted ammunition, but also in Kursk, the first Russian territory to be under foreign military occupation since World War II.

Pyongyang already provides Moscow with much-needed ammunition, including millions of artillery shells, in exchange for key military technology for its nuclear and ballistic missile programs, US officials say.

Moscow and Pyongyang deny any arms transfers but committed to strengthening their military ties in June, when Russian President Vladimir Putin and North Korean leader Kim Jong Un signed a pact in Pyongyang that included a pledge of mutual defense.

Previously, the North Korean military was unable to get real-time feedback on its weapons, said Gen. Charles Flynn, the commander of the U.S. military in the Indo-Pacific region.

“They don’t have a very good reach into North Korea,” he said Tuesday at the Center for a New American Security in Washington.

That could change if North Korean soldiers become involved in the war in Ukraine.

“That kind of feedback from a real battlefield to North Korea to be able to make adjustments to their weapons, their munitions, their capabilities and even their people – I find it very concerning,” Flynn said.

Anastasiia Parafeniuk reported from Kiev, Ukraine, and Mithil Aggarwal reported from Hong Kong.