Vladimir Putin, the president of Russia, and Kim Jong Un, the leader of North Korea, are about to start negotiations on a potential arms trade.
In a journey that was closely monitored, Mr. Kim traveled for two days in an armored train to the Vostochny space center in Russia's far east.
A ballistic missile was fired by Pyongyang hours before the meeting.
It is expected that the two sanctioned regimes would reach an agreement exchanging weapons for humanitarian aid.
Although Pyongyang and Moscow maintain that their talks are for military collaboration, the US and its allies are closely monitoring the meeting between Mr. Kim and Mr. Putin.
The White House claimed to have knowledge of "actively advancing" negotiations between Russia and North Korea on a weapons deal.
National Security Council spokesman John Kirby said Russia's Defence Minister Sergei Shoigu had tried to "convince Pyongyang to sell artillery ammunition" to Russia during a recent visit to North Korea.
North Korea also wants food aid and possibly technology to help its banned nuclear and missiles programme, analysts say.
But the Kremlin said on Tuesday that the "fully fledged visit" would cover "bilateral relations, the situation in the region and in the global arena".
According to North Korean official media, Mr. Kim claimed that his visit demonstrates the "strategic importance" of relationships between Pyongyang and Moscow.
According to Dmitry Peskov, a spokesman for the Kremlin, Russia will always act in its own best interests.
He was quoted by Russian media as saying, "The interests of our two countries are important to us, and not warnings from Washington."
Vladivostok, where Mr. Putin was hosting an economic meeting, was supposed to be the location of the much awaited visit, but Mr. Kim's train traveled by the city on its way to the space center instead.
The Vostochny space center, the most technologically advanced space center in Russia, is rumored to be President Putin's favorite project.
Russia could be approached by North Korea for assistance with its space program. After a rocket failure last month, an attempt to launch a spy satellite into orbit failed for a second time.
On Tuesday morning, Mr. Kim stopped briefly at the border post of Khasan en route to the space center. There, he was greeted by a Russian delegation while a brass band played.
There are rumors that Mr. Kim's train has at least 20 massive, bulletproof vehicles. Due to the added weight, the train can only go at a speed of about 37 mph (59 km/h).
The North Korean leader's most recent travel outside of the country was to Vladivostok in 2019 for a summit with Mr. Putin following the failure of nuclear disarmament negotiations with the then-US president, Donald Trump.