North Korean leader Kim Jong-un, left, toasts Russian President Vladimir Putin during a post-summit dinner at the Far Eastern Federal University in Vladivostok, Russia, in this April 25, 2019 file photo. Yonhap

North Korean leader Kim Jong-un sent a letter to Russian President Vladimir Putin on his 70th birthday praising his leadership against "challenges and threats" from the United States, Pyongyang's state media said Friday.

In the letter, Kim also lauded Putin's "signal achievements in attaining the grand strategic target for building powerful Russia" through his "energetic activities," according to the official Korean Central News Agency (KCNA).

"Today, Russia is reliably defending the dignity of the state and its fundamental interests from the challenges and threats by the U.S. and its vassal forces," he said. "Such reality is unthinkable without your distinguished leadership and strong will."

Kim added that the mutual support and cooperation between Pyongyang and Moscow have been "strengthened as never before" in the struggle to develop traditional bilateral ties under the agreement made at their first summit in Russia's far eastern city of Vladivostok in 2019.

He then expressed hope that their personal ties will play a bigger role in steadily developing relations between the two countries.

North Korea has recently stressed its close ties with Russia amid frosty inter-Korean ties and a drawn-out impasse in nuclear negotiations with Washington. (Yonhap)