South Korea’s intelligence agency said on Friday that North Korea had dispatched troops to assist Russia in its war against Ukraine, a development that could intensify the standoff between North Korea and the west. In a statement on its website, the National Intelligence Service (NIS) said Russian navy ships transferred 1,500 North Korean special operation forces to the port city of Vladivostok between 8 and 13 October who were now undergoing training. “The North Korean soldiers … are expected to be deployed to the frontlines as soon as they complete their adaptation training,” the agency said, adding that more North Korean troops were expected to be sent to Russia soon. NIS said North Korean soldiers were given Russian military uniforms and Russian-made weapons and were issued with fake ID cards of residents of Yakutia and Buryatia, two regions in Siberia. “It appears that they disguised themselves as Russian soldiers to hide the fact that they were deployed to the battlefield,” the agency said. The NIS also published satellite and other photos showing what it calls Russian navy ship movements near a North Korean port and suspected North Korean mass gatherings in the past week in the far-eastern Russian cities of Ussuriysk and Khabarovsk. The statement was the most comprehensive official report to date detailing North Korean involvement in Russia’s war in Ukraine. If proved accurate, it would amount to North Korea’s first major participation in a foreign war. Additionally, South Korean media said on Friday, citing anonymous sources, that Pyongyang had decided to dispatch a total of 12,000 troops, formed into four brigades, to Russia. The NIS did not immediately confirm these reports. The statements come a day after Ukraine’s president, Volodymyr Zelenskyy, said his country had intelligence reports that 10,000 North Korean soldiers were preparing to enter the war. “This is the first step to a world war,” he told reporters in Brussels. On Friday the Ukrainian foreign minister, Andrii Sybiha, demanded an “immediate and strong reaction” from Kyiv’s allies. “Russia seriously escalates its aggression by involving DPRK [North Korea] on a war party scale,” he said. “We require an immediate and strong reaction from the Euro-Atlantic community and the world.” Russia has denied using North Korean troops in the war. A Kremlin spokesperson, Dmitry Peskov, dismissed the claims at a media conference last week as “another piece of fake news”. The Nato secretary general, Mark Rutte, said on Friday that the alliance could not yet confirm South Korean intelligence that North Korea was deploying large-scale troops. The Guardian earlier revealed that North Korean military engineers had already been deployed to help Russia target Ukraine with ballistic missiles. A source in Ukraine said: “There are dozens of North Koreans behind Russian lines, in teams that support launcher systems for KN-23 missiles.” If confirmed, South Korean intelligence suggests that North Korea plans to engage in the war beyond merely sending military advisers. The extraordinary move also highlights Russia’s need to find new military personnel, amid reports of record casualties. North Korea’s leader, Kim Jong-un, and Russia’s president, Vladimir Putin, who first met in 2019, have been seeking greater military and economic cooperation to counter their growing international isolation prompted by Russia’s invasion of Ukraine and North Korea’s ballistic missile and nuclear weapons programmes. In June, the two leaders signed a pact that includes a clause requiring the countries to come to each other’s aid if either is attacked. Pyongyang is estimated to have provided about half the larger-calibre ammunition used on the battlefield this year, more than 2m rounds, a Ukrainian source said. It also provided KN-23 missiles, which were used in dozens of strikes across Ukraine last winter, Ukrainian media reported. In return for its missiles and other military hardware, North Korea is thought to be seeking Russian cash as well as help with its spy satellite programme, which has had embarrassing failures over the past two years.