North Korea's Kim Jong Un and Vladimir Putin have discussed "possibilities" for military cooperation, in a highly scrutinised meeting for a suspected arms deal.
The pair met on Wednesday at the Vostochny space centre after Mr Kim arrived in his private armoured train.
Mr Putin also said he would help Pyongyang develop satellites.
The US says Moscow is buying weapons for its war on Ukraine, and warned any help would violate UN resolutions.
Mr Putin has also accepted an invitation from Mr Kim to visit North Korea, according to Pyongyang. Few heads of state have visited the closed-off state.
Wednesday's meeting between the two sanctioned regimes, which included senior officials from both sides, took place at a time when their relations with the West are at an all-time low.
Mr Kim was warmly received by Mr Putin after travelling for two days to Russia's far east. Russian state media footage showed the two leaders grinning as they shook hands, before Mr Putin personally escorted Mr Kim around the space centre.
Citing historical ties between the Soviet Union and North Korea, Mr Putin welcomed his counterpart with the Russian proverb "an old friend is better than two new ones".
Asked if Russia would help North Korea build satellites, Mr Putin said "this is why we've come to Vostochny Cosmodrome", Russian media reported.
Mr Putin also said they would "discuss all topics" when asked if he would talk to Mr Kim about a weapons deal.
Meanwhile, Mr Kim appeared to express support for Mr Putin's war in Ukraine.
"Russia has risen to a sacred fight to protect its sovereignty and security against the hegemonic forces" of the West, Mr Kim told Mr Putin.
"We will always support the decisions of President Putin and the Russian leadership... and we will be together in the fight against imperialism."
The North Korean leader is expected to oversee a display of Russian warships later, as well as visit several factories and stop by the eastern city of Vladivostok on his way home. It is not known how long he will stay in Russia.
Earlier this year North Korea twice tried, and failed, to launch a spy satellite. Pyongyang has vowed to develop one to boost military surveillance.
But the US believes North Korea's satellite programme is also aimed at boosting its ballistic missile capabilities, as the technology is similar.
On Wednesday US State Department spokesman Matthew Miller, in response to reporters' queries, agreed there was a concern that Russian help with satellite technology would actively improve the North Korean missile programme.
"That is quite troubling and would potentially be in violation of multiple UN Security Council resolutions" which Russia itself had voted for in the past, he said.
Mr Putin appeared to acknowledge this on Wednesday, saying there were "there are certain limitations" to military co-operation.
The US has also warned that it would "not hesitate to take action to hold those accountable if necessary", to which the Kremlin had responded that the interests of Russia and North Korea were important to them "and not warnings from Washington".
The meeting marked Mr Kim's first trip abroad since 2019. The last time he travelled outside North Korea was also to meet Mr Putin after the collapse of North Korea's nuclear disarmament talks with then US president Donald Trump.
Many had expected him to head to Vladivostok where Mr Putin was attending an economic forum, but instead the train chugged northwards to Vostochny. On Wednesday morning, as Mr Kim neared his destination, North Korea fired two short-range missiles into the sea off its east coast, the latest in a series of banned weapons tests.
Mr Kim and Mr Putin's meeting follows a Russian delegation's visit to North Korea in July, where Mr Kim showed off Pyongyang's missiles, including the Hwasong intercontinental ballistic missile, to defence minister Sergei Shoigu.
Moscow would be keen on North Korean arms due to their compatibility with Russian weapon systems, say experts.
They would be particularly eager for artillery shells and guns as artillery is "the god Russia worships" on the battlefront, said Valeriy Akimenko, an expert on Russia's military with the Conflict Studies Research Centre.
Pyongyang would likely oblige in providing these as well as bullets and "even older types of missiles", said Kim Dong-yup, a professor at the University of North Korean Studies.
Yang Uk, a research fellow at the Asian Institute for Policy Studies, said it is also possible that newer weapons such as short-range ballistic missiles could be supplied, such as the so-called "super-large" rocket KN-25.
Some analysts believe North Korea could have a large stockpile of arms as it has not fought a war since the Korean War ended in armistice in 1953, though others think Pyongyang may be reluctant to hand over too much given their relative lack of resources.
But observers also say that North Korean weapons would only give a short-term boost to Russia's war effort. They point to how Moscow, with hugely depleted ammunition, is relying on older, more unreliable artillery shell stocks.
North Korea's arms could act "as a stop-gap measure" while Russia struggles to ramp up production, noted Mr Akimenko.
But given how fast Russia has been going through its supplies, the deal would not have much impact strategically. "It would kill more Ukrainians. But it will not kill Ukraine," he added.
In return, Mr Kim is thought to be asking for food aid for his impoverished country.
North Korea, which has long struggled under sanctions, has been especially hit hard by border closures during Covid which it has only recently started relaxing.
It may also ask for more advanced submarine and ballistic technology from Russia - though Mr Putin may draw a line at that, say some observers.
"Even a desperate war machine does not trade its military crown jewels for old, dumb munitions," said Leif-Eric Easley, a professor at Ewha Womans University in Seoul.
A deeper question posed by the meeting is whether heavy sanctions on Russia and North Korea are really working.
Rorry Daniels, managing director of the Asia Society Policy Institute, said their meeting showed that international sanctions have created a "firewall" where the two countries "can transact business without fear of further punishment".
"The more states under severe sanction are pushed together, the less the US can do to use sanctions as leverage to resolve the underlying conflicts."
But the situation is also not without risk for Pyongyang, noted Park Won-gon, an associate professor in North Korean studies at Ewha Womans University.
If any evidence emerges indicating that North Korean weapons were used by Russia in Ukraine, "it may result in North Korea turning the entire Nato alliance against it, which could subsequently trigger additional sanctions."
Additional reporting by Yuna Ku based in Seoul.