The Kremlin confirmed the trip on Thursday after the two men held a rare summit in Russia's Far East.

Mr Kim is currently in Russia, where he has vowed to support the Kremlin's "sacred war" against Ukraine, raising concerns the two nations could bolster their military capabilities.

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"At the end of the reception, Kim Jong Un courteously invited Putin to visit the DPRK at a convenient time," KCNA reported, referring to the Democratic People's Republic of Korea, the country's formal name.

The two leaders toasted their friendship on Wednesday with Russian wine after President Putin showed Mr Kim around the country's most modern space launch facility.

They also held talks alongside their defence ministers and called each other "comrades".

Kremlin spokesman Dmitry Peskov also confirmed Russian Foreign Minister Sergei Lavrov will visit Pyongyang next
month.

The burgeoning friendship has concerned the West, with Washington accusing North Korea of providing weapons to Russia.

It is not clear if any deliveries have been made and the two states have denied those claims in any case - but they did promise to deepen defence cooperation.

Mr Kim is due on Thursday to visit military and civilian aviation factories in the Russian city of Komsomolsk-on-Amur and to inspect Russia's Pacific fleet in Vladivostok, Mr Putin said.

What kind of weapons could North Korea provide?

Aidan Foster-Carter, a researcher in sociology and modern Korea at Leeds University, told Sky News that North Korea can probably supply "good quality ammunitions" to Russia.

He said Mr Putin and Mr Kim each had something the other wanted - but it was still unclear exactly what that was going to be.

Russia needs artillery shells, he said, and he is "betting" Mr Kim needed money, food and cheap oil.

Asked how Mr Kim could help Mr Putin, he said North Korea was a "highly-militarised state" and focused on producing armour.

"They probably can supply good quality ammunitions," he said.

Growing criticism

If an arms deal was to be reached, the US warned it "won't hesitate" to slap additional sanctions on the two countries.

South Korea, meanwhile, expressed "deep concerns" over potential military cooperation.

Unification minister Kim Young-ho, who oversees relations with the North, expressed "deep concerns", adding Moscow and Pyongyang were apparently chasing "some kind of" a military deal.

But Russian diplomats dismissed criticism and accused the US of hypocrisy as its own policies had sent weapons to allies around the world.

"The United States has no right to lecture us on how to live," Russia's ambassador to the United States, Anatoly Antonov, said in a statement.

Toast to victory for 'great Russia'

Despite growing international criticism, Mr Kim on Wednesday toasted their friendship and to the victory of "great Russia" over a lunch of Russian pelmeni dumplings, white Amur fish soup and sturgeon.

North Korea was founded in September 1948 with the backing of the Soviet Union, and Moscow supported it for decades during the Cold War.

Support did drop after the Soviet Union collapsed in 1991, but Mr Putin visited Pyongyang in 2000 - a year after taking over from Boris Yeltsin - for a meeting with Mr Kim's father, Kim Jong Il.

China has been seen as the greatest influence over Mr Kim in recent years, but North Korea often tries to balance ties between Moscow and Beijing.