Seoul: South Korea and the US on Tuesday began a four-day joint navy exercise in South Korean waters amid mounting tensions with North Korea. The joint maritime exercise will be conducted in the Yellow Sea and the Sea of Japan and will include 50 warships from both countries, a South Korean navy spokesperson told EFE news agency. The navies will conduct special maritime operations and live-fire drills against mock hostile submarines, aircraft and warships till Friday. The new exercises are part of the large-scale Foal Eagle manoeuvre, which began on March 7 and will continue till April. These annual military exercises are the largest ever conducted in the Korean Peninsula and involve more than 17,000 American and 300,000 South Korean troops. North Korea, which considers these joint military exercises "invasion drills", responded with threats of pre-emptive strikes and launched several short and medium range missiles, including one on Monday. The US has a permanent force of around 28,500 troops in South Korea to defend it against North Korean aggression in an arrangement dating back to the end of the Korean War (1950-53).