(Reuters) - Kim Jong Un invited Kremlin chief Vladimir Putin to visit North Korea during a rare summit, stoking U.S. concerns that a revived Moscow-Pyongyang axis could bolster Russia's military in Ukraine and provide Kim sensitive missile technology.

Putin accepted the invitation, according to North Korean state news agency KCNA, though there was no immediate confirmation from the Kremlin. Since Russia's 2022 invasion of Ukraine, Putin has rarely travelled abroad.

Calling each other "comrades", Putin and Kim toasted to their friendship on Wednesday with Russian wine after the 70-year-old Russian president showed Kim, 39, around Russia's most modern space launch facility and they held talks alongside their defence ministers.

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

"Putin accepted the invitation with pleasure and reaffirmed his will to invariably carry forward the history and tradition of the Russia-DPRK friendship," KCNA said.

For the United States and allies, the burgeoning friendship between Kim and Putin is a concern: Washington has accused North Korea of providing arms to Russia, but it is unclear whether any deliveries have been made.

Both Russia and North Korea have denied those claims, but promised to deepen defence cooperation, and during a visit to North Korea in July, Defence Minister Sergei Shoigu was shown banned ballistic missiles by Kim.