North Korean leader Kim Jong Un has set off for Russia aboard a special train, a South Korean source said, as Pyongyang and Moscow on Monday confirmed a summit with President Vladimir Putin amid Russia's deepening isolation over the war in Ukraine.

Kim would visit Russia in the coming days at the invitation of Putin, the Kremlin said, while North Korean state news agency KCNA said the two would "meet and have a talk", without elaborating.

U.S. officials have said the pair would discuss possible arms deals to aid Russia's war in Ukraine and provide North Korea with a much-needed economic and political lifeline.

Washington and its allies have been voicing concern at recent signs of closer military cooperation between Russia and the nuclear-armed North. It will be Kim's second su

The North Korean leader left aboard his train late on Sunday, a senior South Korean government official told Reuters.

The special train would take Kim to North Korea's northeastern border with Russia and the summit could take place as early as Tuesday, said the official, who spoke on condition of anonymity, citing intelligence concerns, and added that details could change depending on the situation there.

Neither Moscow nor Pyongyang immediately confirmed an exact schedule for the visit.

North Korea is one of the few countries to have openly supported Russia since the invasion of Ukraine last year, and Putin pledged last week to "expand bilateral ties in all respects in a planned way by pooling efforts".

Kim's last trip abroad in 2019 was also to Vladivostok for his first summit with Putin after the collapse of North Korea's nuclear disarmament talks with former U.S. President Donald Trump.

Japanese media reported security was being stepped up and refurbishment was taking place at the main train station in the Russian border city of Khasan, where Kim is expected to enter Russia.

Kim does not travel abroad often and, when he does, it is often shrouded in secrecy and security. He travels by a special train with its signature olive green carriages that are said to be armoured and equipped with communications systems and a personal suite for Kim to work and confer with aides.

A summit between Kim and Putin would likely focus on military cooperation and possibly a deal to supply arms, U.S. and South Korean officials and analysts have said.

North Korea is believed to have a large stockpile of artillery ammunition that would be compatible with Russian weapons and could help supplement the supplies being rapidly used on the Ukrainian frontlines.

The United States has said it would be a "huge mistake" for North Korea to supply Russia with weapons to use in Ukraine and warned Pyongyang would "pay a price".

The deepening relationship between Kim and Putin signals a further global split over the war, said Ramon Pacheco Pardo, the Korea Chair at the Brussels School of Governance.

"North Korea's support will allow Putin to wage his war for longer, which is bad news for Europe," he said. "It is further proof that much of the world doesn't support Ukraine in the way the U.S. and Europe do, and some countries such as North Korea will openly support Russia without fear of any real consequences."

In Vladivostok, there was a higher police presence than usual on the streets but no North Korean flags had been put up - unlike ahead of Kim's previous trip when the city was adorned with the red five pointed stars that grace the North's flag.mmit with Putin, after the pair met in 2019.

Despite denials by both Pyongyang and Moscow, the United States has said talks are advancing actively for North Korea to supply arms to Russia, which has expended vast stocks of weapons in more than 18 months of war.