North Korea may be readying to test fire a medium-range missile to underscore its anger over joint US-South Korea military drills that began this week, a report said Tuesday. Mobile launchers used for Rodong missiles have been deployed at a North Korean missile base north of Pyongyang, South Korea's Yonhap news agency reported, citing military sources. "We've detected signs that North Korea has deployed two transporter erector launchers since a few days back in its Rodong missile station in North Pyongyang Province," one source was quoted as saying. North Korea used similar launchers in March last year to fire two Rodong-class missiles with a maximum range of 1,000-1,500 kilometres, the source said. That test was timed to coincide with US President Barack Obama hosting a landmark Japan-South Korea summit to discuss joint security concerns in the face of Pyongyang's nuclear threat. The South's defence ministry refused to confirm the Yonhap report, but said it was monitoring the situation. "Our troops have been closely watching North Korea for additional (missile) launches," ministry spokesman Kim Min-Seok told reporters. "We've stepped up our surveillance in cooperation with the United States, and put the crisis management system into operation," he said. North Korea fired two short-range Scud missiles into the sea off its east coast on Monday as South Korea kicked of a series of joint military drills. The annual exercises always trigger a surge in military tensions and warlike rhetoric on the divided peninsula. UN resolutions ban any ballistic missile test by North Korea, and Seoul has vowed to respond "sternly and strongly to any provocation". Missile tests have long been a preferred North Korean method of expressing anger and displeasure with what it views as confrontational behaviour by the South and its allies. The largest element of the two South Korea-US drills that began Monday is Foal Eagle, an eight-week exercise involving air, ground and naval field training, with around 200,000 Korean and 3,700 US troops. The other is a week-long, largely computer-simulated joint drill called Key Resolve. Seoul and Washington insist the exercises are defence-based in nature, but they are regularly condemned by Pyongyang as provocative rehearsals for invasion.