Russian President Vladimir Putin met his Vietnamese counterpart Thursday as he began a state visit to Hanoi, a day after signing a mutual defence pact with North Korea's Kim Jong Un.

Putin and Kim inked a strategic treaty at a summit in Pyongyang that included a commitment to come to each other's aid if attacked. Kim also pledged his "full support" for Russia's war on Ukraine.

Washington and its allies accuse North Korea of supplying ammunition and missiles to Russia for its war in Ukraine, and the deal has fuelled fears of more deliveries.

The US State Department said deepening Russia-North Korea ties were "of great concern" while a top Ukrainian official accused Pyongyang of abetting Moscow's "mass murder of Ukrainians".

Putin was welcomed by newly installed Vietnamese President To Lam and a military guard at the presidential palace in Hanoi.

He will hold talks with Lam and later meet other senior Vietnamese leaders including Nguyen Phu Trong, the powerful general secretary of the ruling Communist Party.

Support from Kim

Making his first visit to the isolated North in 24 years on Wednesday, Putin said he did not rule out "military-technical cooperation" with Pyongyang, which like Moscow is under heavy international sanctions.

"Today, we are fighting together against the hegemonism and neo-colonial practices of the United States and its satellites," Putin said.

The two countries have been allies since North Korea's founding after World War II and have drawn even closer since Russia's invasion of Ukraine in 2022 isolated Putin on the global stage.

Kim called Putin the "dearest friend of the Korean people" and vowed him his "full support and solidarity" over the war in Ukraine, which has triggered rafts of UN sanctions on Moscow.

Putin thanked his host -- whose country has been under a UN sanctions regime since 2006 over his banned weapons programmes -- saying Moscow appreciated the "consistent and unwavering" support.

Putin called for a review of UN sanctions on North Korea and said the two countries would not submit to Western "blackmail".

Reacting to the Pyongyang visit, a US State Department spokesperson said no country should "give Mr Putin a platform to promote his war of aggression against Ukraine".

"Deepening cooperation between Russia and the DPRK is a trend that should be of great concern to anyone interested in maintaining peace and stability on the Korean Peninsula," the spokesperson said.

Mykhailo Podolyak, a senior aide to Ukraine's President Volodymyr Zelensky, told AFP that North Korea was abetting Russia's "mass murder of Ukrainians", and called for greater international isolation of both countries.

Trade and arms

Putin received a rapturous reception in the North Korean capital, embraced by Kim as he stepped off his plane and greeted by cheering crowds, synchronised dancers and flag-waving children.

His reception was more reserved in Vietnam, a major global manufacturing hub that has carefully hedged its foreign policy position for years, seeking to be friends with all but beholden to none.

In particular, it has sought to avoid picking sides in the growing US-China rivalry even as both superpowers look to boost their influence in Southeast Asia.

US President Joe Biden visited Hanoi in September to promote ties as his administration seeks to build up Vietnam as an alternative supplier of key high-tech components to reduce American dependence on China.

Beijing swiftly followed suit, with President Xi Jinping making his own state visit barely three months later.

Russian officials say Putin's visit will focus on economic, education and energy issues.

Trade between the two countries stood at just $3.5 billion in 2022 -- a tiny fraction of Vietnam's $175 billion trade with China and $123 billion with the United States.

But observers say that in private, Ukraine and defence cooperation are likely to be on the table.

Vietnam has abstained in UN votes condemning the Russian invasion, and in an op-ed in the communist party mouthpiece Nhan Dan newspaper, Putin thanked Hanoi for its "balanced stance on the Ukraine crisis".

Russia and Vietnam have deep ties dating back to the 1950s, and for decades, Moscow was Hanoi's leading arms supplier. 

Carl Thayer, emeritus professor of politics at Australia's University of New South Wales, said Vietnam has stopped "big ticket" military purchases since 2022, when Russia invaded Ukraine.

"Russia and Vietnam have a mutual interest in resuming arms sales but Vietnam is hamstrung by the threat of US sanctions," he told AFP.