A South Korean marine LVT-7 landing craft sail to shores through a smoke screen during the U.S.-South Korea joint landing exercises in Pohang, South Korea, Monday. / Ahn Young-joon, AP

by Hyung-Jin Kim and Jung-Yoon Choi, Associated Press

by Hyung-Jin Kim and Jung-Yoon Choi, Associated Press

SEOUL, South Korea (AP) - 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.

The North in recent weeks has increased threatening rhetoric and conducted a series of rocket and ballistic missile launches that are considered acts of protest against annual ongoing springtime military exercises by Seoul and Washington. The North calls the South Korea-U.S. drills a rehearsal for invasion; the allies say they're routine and defensive.

Pyongyang threatened Sunday to conduct a fourth nuclear test at some point, though Seoul says there are no signs of an imminent detonation. Wee Yong-sub, a deputy spokesman at the South Korean Defense Ministry, said the North Korean warning about the live-fire drills Monday was a "hostile" attempt to heighten tension on the Korean Peninsula.

A woman who runs a lodging facility on another front-line island, Baengnyeong, said from a shelter that she was still hearing the sounds of artillery fire about 90 minutes after the North began its live-fire drills.

The western sea boundary has been the scene of several bloody naval skirmishes between the Koreas in recent years, including the 2010 artillery attack by the North.

Last spring, tension spiked after a near-daily barrage of North Korean threats, including warnings of nuclear strikes against Seoul and Washington, following international criticism of Pyongyang's third nuclear test in February of last year. The North has since gradually dialed down its threats and sought improved ties with South Korea in what foreign analysts say is an attempt to lure international investment and aid. There has been no major breakthrough in the North's reported push to win outside aid, however, with Washington and Seoul calling on the North to first take disarmament steps to prove its sincerity about improving ties, analysts say.

The North Korean live-fire drills and the country's hints at a nuclear test are meant to express anger and frustration over what the North sees as little improvement in progress in its ties with South Korea and the U.S., said Lim Eul Chul, a North Korea expert at South Korea's Kyungnam University. Lim said the North might conduct a fourth nuclear test and launch other provocations to try to wrest the outside concessions it wants.

The Korean Peninsula remains in a technical state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty. About 28,500 American troops are deployed in South Korea to deter potential aggression from North Korea.