This pool photograph distributed by Russian state owned agency Sputnik shows Russia's Foreign Minister Sergei Lavrov attending a meeting of presidents of Russia and Kyrgyzstan in Bishkek, Oct. 12. AFP-Yonhap

Russian Foreign Minister Sergei Lavrov is due to arrive in Pyongyang on Wednesday for meetings seen as setting the stage for a visit by President Vladimir Putin, who has stepped up cooperation with politically isolated North Korea.

Lavrov's two-day visit comes a month after North Korean leader Kim Jong Un made a rare trip to Russia, during which he invited Putin to Pyongyang and discussed military cooperation, including over North Korea's satellite program and the war in Ukraine.

Russia's TASS news agency reported that Lavrov may brief the North Koreans on the results of Putin's visit to China, as well as discuss the standing invitation to visit Pyongyang.

U.S. Special Representative for North Korea Sung Kim on Tuesday called relations between North Korea and Russia "worrying," after the White House said last week Pyongyang recently provided Russia with weapons. A growing number of reports by the U.S. government and Western researchers have documented with satellite imagery what they say are North Korean weapons shipments to Russia for use in the war in Ukraine. Kremlin spokesman Dmitry Peskov said on Tuesday that the Western allegations were not based on evidence, TASS reported.

On Monday the U.K.-based Royal United Services Institute released dozens of high-resolution commercial satellite images that it said showed two Russian ships with connections to the country's military logistics networks making multiple trips to North Korea.

The two ships had moved several hundred containers to and from a port in North Korea since August, the RUSI report said.

Although acknowledging it was impossible to confirm their contents, the report said containers of the same size and colour were later seen being delivered to a recently expanded Russian munitions storage facility near the border with Ukraine.

North Korea is heavily sanctioned over its nuclear weapons and ballistic missile programs, and United Nations Security Council resolutions ― approved at the time with Russia's support ― ban cooperation with Pyongyang on military issues as well as in a range of other areas. (Reuters)