Russian President Vladimir Putin and North Korean leader Kim Jong Un will meet Wednesday in Pyongyang for a summit where they are widely expected to discuss ways to boost sanctions-busting military ties.
Kim greeted Putin at the airport, with the two internationally isolated leaders hugging on the red carpet, photographs in North Korean state media showed, underscoring the increasingly close relationship that has sparked concern in Seoul and Washington.
It is Putin's first trip to North Korea since 2000 and comes as Western countries accuse Kim of supplying arms to historic ally Russia for use in Ukraine in violation of UN sanctions on both countries, allegations Moscow and Pyongyang have officially denied.
Kim was shown standing on the red carpet waiting for Putin to emerge from his private plane in the wee hours, before escorting the Russian leader to his waiting car.
The Korean Central News Agency reported that Kim "shook hands with Putin and embraced him warmly, expressing his joy and gladness to meet him again" following a summit in Russia's far east last year.
The visit highlights the "invincibility and durability" of the bilateral relationship, KCNA said, with the leaders expected to hold talks later Wednesday, after an official welcoming ceremony.
Putin -- who is travelling with a slew of Russian officials including Foreign Minister Sergei Lavrov -- spoke with Kim at the airport and during a motorcade ride, KCNA said.
The Kremlin released a document on Tuesday confirming Russian plans to sign a "strategic partnership" treaty with North Korea.
Experts have warned the trip is likely to focus on defence ties, although publicly the leaders are expected to highlight cooperation in the economic sector as any arms deals would violate United Nations Security Council resolutions banning Pyongyang's nuclear and ballistic missile programs.
Pyongyang has described allegations of supplying weapons to Russia as "absurd".
However, it thanked Russia in March for using its Security Council veto to effectively end monitoring of sanctions violations just as UN experts were starting to probe alleged arms transfers.
"Moscow and Pyongyang will likely continue to deny violations of international law but have notably shifted from hiding their illicit activities to flaunting their cooperation," said Leif-Eric Easley, a professor at Ewha University in Seoul.
The visit is a way for Putin to thank the North "for acting as an 'arsenal for autocracy' in support of his illegal invasion of Ukraine," he said.
The fact that Russia's leader has come to the North is "politically important because it allows Pyongyang's propaganda to portray Kim as a world leader," he added.
Russia needs ammunition for its war in Ukraine, and North Korea is eager for high-end military technology to advance its nuclear, missile, satellite and nuclear-powered submarine programmes, according to experts.
But any transfers of "sensitive military technologies to Pyongyang would not only violate UN sanctions but could also destabilize the Korean Peninsula and East Asia," Easley added.
The United States voiced "concern" Monday about Putin's trip over the security implications for South Korea as well as Ukraine.
The two Koreas have remained technically at war since their 1950-53 conflict and the border dividing them is one of the most heavily fortified in the world.
Highlighting those security concerns, South Korea said its troops fired warning shots at soldiers from the North who briefly crossed the border Tuesday and then retreated.
The South's military said it believed the North Korean soldiers accidentally crossed as they were fortifying the border, but said some were wounded after detonating landmines.
