South Korea and the United States started a joint military drill this morning, amid threats of military retaliation from North Korea. About 2 lakh South Korean and 13,000 US troops are taking part in the drill, named Key Resolve and Foal Eagle. South Korea has described the drill as a routine annual defense exercise. But Pyongyang says the drill is a pretext for an invasion of North Korea and has warned it will respond with an all-out war that would turn South Korea's capital, Seoul, into a sea of fire. North Korea has also threatened to open fire across the tense land border between the two Koreas, if the South continues to launch propaganda balloons. The balloons recently have been carrying leaflets about the uprising in Egypt, a news that is banned in North Korea.