North Korean leader Kim Jong Un on Wednesday hailed his country's ties with Russia, saying the two nations were "invincible comrades-in-arms" amid reports President Vladimir Putin will visit Pyongyang this summer.Kim made a rare overseas trip to meet the Kremlin leader in Russia's far east last year, with Seoul and Washington subsequently accusing Pyongyang of shipping weapons to Moscow for use on the battlefield in Ukraine.Putin is expected to visit North Korea in the coming weeks after the Kremlin told Russian media in May that the trip was "being prepared."South Korea's Yonhap news agency reported the visit could take place "as early as next week," as part of a tour that would also include a stop in Vietnam.Ties between North Korea and Russia have "developed into an unbreakable relationship of comrades-in-arms," leader Kim wrote in a message to Putin carried by the official Korean Central News Agency on Wednesday.Their "meaningful" ties will "further consolidate the eternal milestone" in the new era, Kim added, according to KCNA.Yonhap said satellite images showed possible signs of a "large structure" being installed in Pyongyang's Kim Il Sung Square.

This kind of activity has previously been observed when North Korea was preparing for large-scale events in the capital, such as military parades or visits by high-ranking foreign figures.During his trip to Russia last year, Kim said ties with Moscow were his country's "number one priority."Analysts have warned that a recent increase in testing and production of artillery and cruise missiles by the nuclear-armed North could be in preparation for shipments to Russia. Pyongyang has denied the allegations, calling them "absurd."North Korea is barred by UN sanctions from any tests using ballistic technology, but Moscow used its UN Security Council veto in March to effectively end UN monitoring of violations, for which Pyongyang has specifically thanked Russia.A Pentagon report published last month said Russia is using North Korean ballistic missiles in Ukraine, citing debris analysis.

Experts said that during Putin's visit to Pyongyang, North Korea would likely push to export more weapons in exchange for importing food and energy from Russia.