North Korean leader Kim Jong Un stopped in a far eastern Russian city Friday to see a factory that builds the country’s most advanced fighter jets on his extended trip that hints at his interest in sophisticated weaponry, as the U.S. and others warned Moscow and Pyongyang against making banned weapons transfer deals.

Kim’s visits to Russian weapons and technology sites and meetings with President Vladimir Putin have raised speculation he will supply ammunition to Russia for its war efforts in Ukraine in exchange for receiving advanced weapons or technology from Russia as the two nations deepen their ties while both are increasingly isolated and sanctioned in separate confrontations with the West.

Russia’s state news agency RIA Novosti published video showing Kim’s armored train pulling into a station in the city of Komsomolsk-on-Amur and Kim’s convoy sweeping out of the station shortly afterward. TASS news agency said Kim and local Russian officials were headed for a plant that produces Su-35 and Su-57 fighter jets.

Kim is to travel next to Vladivostok to view Russia’s Pacific fleet, a university and other facilities, Putin told Russian media after his summit with Kim.

Experts say in return for helping Putin replenish war supplies, Kim would seek Russian help to modernize his air force and navy, which are inferior to those of rival South Korea while Kim has devoted much of his own resources to his nuclear weapons program.

The summit between Kim and Putin this week took place at the Vostochny Cosmodrome, Russia’s most important domestic launch center. North Korea has struggled to put into space an operational spy satellite to monitor U.S. and South Korean military movements. Asked whether Russia will help North Korea obtain satellites, Putin said “That’s why we have come here. (Kim) shows keen interest in rocket technology. They’re trying to develop space, too,” according to Russian state media. Putin, for his part, would want to receive ammunition, artillery shells and even ballistic missiles from North Korea to replenish his exhausted arms inventory in the second year of Russia’s war in Ukraine, foreign experts say.

Since last year, the U.S. accused North Korea of providing ammunition, artillery shells and rockets to Russia, likely much of them copies of Soviet-era munitions. South Korean officials said North Korean weapons provided to Russia have already been used in Ukraine.

On Thursday evening, the national security advisers of the U.S., South Korea and Japan talked by phone and expressed “serious concerns” about prospective weapons deals between Russia and North Korea. They warned Russia and North Korea would “pay a clear price” if they go ahead with such deals, according to South Korea’s presidential office.

The White House said the three national security advisers noted that any arms export from North Korea to Russia would directly violate multiple U.N. Security Council resolutions, including resolutions that Russia, a permanent member of the U.N. council, itself voted to adopt. They reiterated their cooperation toward the complete denuclearization of the Korean Peninsula as well, according to a White House statement.

South Korean Unification Minister Kim Yung-ho warned Thursday that potential arms transfers between the North and Russia would invite stronger responses from South Korea, the U.S. and Japan, which have been stepping up their trilateral security cooperation to cope with regional threats.

Some analysts question how much Russia would be willing to share its closely guarded high-tech weapons technologies with North Korea in return for its conventional arms. But others say Russia would so because of its urgent need to refill its drained reserves.

Putin told reporters that Russia and North Korea have “lots of interesting projects” in spheres like transportation and agriculture and that Moscow is providing its neighbor with humanitarian aid. But he avoided talking about military cooperation, saying only that Russia is abiding by the sanctions prohibiting procuring weapons from North Korea.

North Korea’s state media said on Thursday that Kim invited Putin to visit North Korea at a “convenient time.” Kremlin spokesman Dmitry Peskov later said that Putin had accepted the invitation and that Russian Foreign Minister Sergey Lavrov is expected to visit North Korea in October.

During Wednesday’s summit, Kim vowed “full and unconditional support” for Putin in what he described as a “just fight against hegemonic forces to defend its sovereign rights, security and interests,” in an apparent reference to the war in Ukraine.

Information on Kim’s trip to Russia is largely from the two nations’ official media outlets. North Korean state media did not provide updates Friday on Kim’s activities. They typically report on Kim’s activities the day after the occur, apparently to align with North Korea’s propaganda needs to glorify Kim.