SEOUL, South Korea — North and South Korea fired artillery shells into each other’s waters Monday, a flare-up of animosity between the rivals that forced residents of five front-line South Korean islands to evacuate to shelters, South Korean officials said. The South Korean artillery fire came after shells from a North Korean live-fire drill fell south of the Koreas’ disputed western sea boundary, an official with South Korea’s Joint Chiefs of Staff said. No shells from either side were fired at any land or military installations, said the official, who provided no other details and spoke on condition of anonymity because of office rules.The exchange of fire followed Pyongyang’s earlier, unusual announcement that it would conduct live-fire drills in seven areas north of the poorly marked Yellow Sea boundary between the countries. North Korea routinely test-fires artillery and missiles into the ocean, but it’s rare for the country to disclose such training plans in advance. The announcement was seen as an expression of Pyongyang’s frustration at making little progress in its recent push to win outside aid. In addition to sending residents of five front-line South Korean islands to shelters, Lee Han-seok, an official with Ongjin county, which governs the islands, also said that ferry service linking the islands to the mainland was stopped.Kang Myeong-sung, speaking from a shelter on Yeonpyeong island, which is in sight of North Korean territory, said he hadn’t seen any fighter jets but heard the boom of artillery fire. In 2010, North Korean artillery killed four South Koreans on Yeonpyeong. Pyongyang said it was responding to earlier South Korean drills in disputed waters.