South Korea returns fire into North Korean waters

South Korea on Monday returned fire into North Korean waters after shells from a North Korean live-fire drill fell south of the rivals' disputed western sea boundary, a flare-up that forced residents of five front-line South Korean islands to evacuate to shelters, South Korean officials said.

No shells from either side were fired at any land or military installations, an official with South Korea's Joint Chiefs of Staff said. He 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 the drills, a move 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, an island that 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.

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 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.

After the North's earlier announcement Monday that it would conduct firing drills in seven areas north of the sea boundary, South Korea responded that it would strongly react if provoked.

The international community also responded to the threats. A White House official on Monday called North Korea's recent actions "dangerous and provocative."

White House spokesman Jonathan Lalley said North Korea's continuous threats and provocations would only deepen its isolation. He added that the U.S. remains steadfast in its commitment to the defense of its allies and is working in close coordination with South Korea and Japan.

Russia's Foreign Ministry said it was worried by North Korea's announcement that it may conduct a new nuclear test.

Pyongyang routinely test-fires artillery and missiles into the ocean, but it's rare for the country to disclose such training plans in advance. Wee Yong-sub, a deputy spokesman at the South Korean Defense Ministry, said the North Korean message was a "hostile" attempt to heighten tension on the Korean Peninsula.

The poorly marked western sea boundary has been the scene of several bloody naval skirmishes between the Koreas in recent years.

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

Al Jazeera and The Associated Press