WASHINGTON (NEXSTAR) -- Officials confirm Ukraine has fired on North Korean troops for the first time since thousands of them joined Russia in its war. Ukrainian officials said the North Korean soldiers are wearing Russian uniforms and training on drones, but U.S. officials are still investigating the developments. "We're looking into those," said Pentagon Press Secretary Maj. Gen. Pat Ryder.  Ryder did confirm more North Korean troops are stationed at the Russia-Ukraine border. "Placing these additional, you know, ten to 11 to 12,000 forces in Kursk is definitely something from a combat capability standpoint that could be significant," he said.  Ryder reiterated what it would mean if North Korean troops fought Ukrainians. "They would become legitimate military targets," he said. That's the unified message coming out of Washington, including from the U.S. State Department. "We continue to be greatly concerned about the partnership between North Korea and Russia," said State Spokesperson Matthew Miller.  Last week, top Russian and North Korean diplomats met in Moscow, some with President Vladimir Putin. North Korea also test fired a long-range missile designed to reach the U.S., but officials said it didn't have that capability. Meanwhile, the U.S. and South Korea conducted their first-ever joint live-fire exercise using drones.