Russian President Vladimir Putin vowed on Tuesday to deepen trade and security ties with North Korea and to support it against the United States, as he headed to the reclusive nuclear-armed country for the first time in 24 years.

The U.S. and its Asian allies are trying to work out just how far Russia will go in support of North Korean leader Kim Jong Un, whose country is the only one to have conducted nuclear weapon tests in the 21st century.

In a signal that Russia, a veto-wielding member of the United Nations Security Council, is reassessing its entire approach to North Korea, Putin praised Pyongyang for resisting what he said was U.S. economic pressure, blackmail and threats.

In an article published by North Korean state media, Putin praised "Comrade" Kim, and promised to "jointly resist illegitimate unilateral restrictions", to develop trade and strengthen security across Eurasia.

"Washington, refusing to implement previously reached agreements, continuously puts forward new, increasingly stringent and obviously unacceptable demands," Putin said in the article, printed on the front page of North Korea's Rodong Sinmun, the ruling Workers' Party mouthpiece.

"Russia has always supported and will continue to support the DPRK and the heroic Korean people in their opposition to the insidious, dangerous and aggressive enemy."

Putin noted the Soviet Union was the first to recognise the Democratic People's Republic of Korea (DPRK) founded by Kim's grandfather, Kim Il Sung, less than two years before the 1950 Korean War.

North Korean state media also published articles praising Russia and supporting its military operations in Ukraine, calling them a "sacred war of all Russian citizens".

Putin's state visit comes amid U.S. accusations that North Korea has supplied "dozens of ballistic missiles and over 11,000 containers of munitions to Russia" for use in Ukraine. South Korea, a staunch U.S. ally, has raised similar concerns.

The White House said on Monday it was troubled by the deepening relationship between Russia and North Korea. The U.S. State Department said it was "quite certain" Putin would be seeking arms to support his war in Ukraine.

Moscow and Pyongyang have denied arms transfers but have vowed to boost military ties, possibly including joint drills.

Russia is due to outproduce the whole NATO military alliance on ammunition production this year, so Putin's trip is likely aimed at underscoring to Washington just how disruptive Moscow can be on a host of global crises.

Russia in March vetoed the annual renewal of a panel of experts monitoring enforcement of longstanding United Nations sanctions against North Korea over its nuclear weapons and ballistic missile programmes.

PARTNERSHIP AGREEMENT

Putin's foreign policy adviser, Yuri Ushakov, said Russia and North Korea may sign a partnership agreement during the visit that would include security issues.

He said the deal would not be directed against any other country, but would "outline prospects for further cooperation".

The visit will include one-on-one discussions between the two leaders, as well as a gala concert, state reception, honour guards, document signings, and a statement to the media, Russia's Interfax news agency quoted Ushakov as saying.

Russian Defence Minister Andrei Belousov, Foreign Minister Sergei Lavrov, the ministers for natural resources, health, and transport, the heads of the Russian space agency and its railways, and Putin's point man for energy, Deputy Prime Minister Alexander Novak, will be part of the delegation.

Ahead of the visit, North Korea appears to have been making preparations for a possible military parade in downtown Pyongyang, commercial satellite imagery showed.

The summit presents the greatest threat to U.S. national security since the Korean War, said Victor Cha, a former U.S. national security official now with the Center for Strategic and International Studies.

"This relationship, deep in history and reinvigorated by the war in Ukraine, undermines the security of Europe, Asia, and the U.S. homeland," he wrote in a report on Monday.

He urged Washington to work with Europe and other partners to increase economic and diplomatic pressure on Pyongyang, engage with China, and launch a major human rights and information campaign to flood the North with outside media.

North Korea has been under U.N. sanctions for its ballistic missile and nuclear programs since 2006, and those measures have been strengthened over the years.

The Security Council has been divided over how to deal with Pyongyang.

Russia and China say more sanctions will not help and that joint military drills by the United States and South Korea merely provoke Pyongyang. Two years ago, they vetoed a U.S.-led push to impose more U.N. sanctions on North Korea over its renewed ballistic missile launches.

Washington and its Asian allies accuse Beijing and Moscow of emboldening North Korea by shielding it from more sanctions.

After North Korea, Putin will visit Vietnam on June 19-20.