South Korean navy ship sinks

South Korea's military has sent vessels to waters near its maritime border with North Korea after an explosion ripped a hole in the bottom of a South Korean navy ship, sinking it.

The shipwith more than 100 crew on board began sinking off the coast of South Korean-controlled Baengnyeong Island, close to North Korea, around 10:45 p.m. local time Friday, said an official at the Joint Chiefs of Staff office in Seoul, speaking on condition of anonymity.

The exact cause was not immediately clear. South Korea's Yonhap news agency reported an explosion in thestern of the ship, but the official said he could not confirm the report.

Minutes later, Yonhap reported that another South Korean ship fired shots toward an unidentified target in the direction of North Korea. The Joint Chiefs official said he could not confirm the shooting but said the military was strengthening its vigilance in the area.

South Korean President Lee Myung-bak convened an emergency meeting of security-related ministers, Yonhap said.

Six naval ships and two coast guard vessels were rushed to the area to rescue the crew ofthe sinking ship, Yonhap said.At least 58 have been saved so far, the news agency reports.

The incident comes amid heightened tension between the two Koreas, which remain in a state of war because their three-year conflict ended in a truce, not a peace treaty, in 1953.

North Korea warns of 'nuclear strikes'

Earlier Friday, North Korea's military warned South Korea andthe United States on Friday of "unprecedented nuclear strikes" as it expressed anger over a report the two countries plan to prepare for possible instability in the totalitarian country. There were no clear indications that the ship's sinking was tied to the North Korean threat.

The North routinely issues such warnings. Diplomats in South Korea and the U.S. have repeatedly called on Pyongyang to return to international negotiations aimed at ending its nuclear programs.

"Those who seek to bring down the system in the [North], whether they play a main role or a passive role, will fall victim to the unprecedented nuclear strikes of the invincible army," North Korea's military said in comments carried by the official Korean Central News Agency.

The North, believed to have enough weaponized plutonium for at least half a dozen atomic bombs, conducted its second atomic test last year, drawing tighter United Nations sanctions.

The North Korean statement Friday specifically referred to the March 19 newspaper report. A spokeswoman said the South Korean Defence Ministry had no information.

China, Japan, Russia, South Korea and the U.S. have been trying to persuade North Korea to give up its nuclear weapons in six-party talks. The North quit the negotiations last year.

The fate of the North's nuclear weapons has taken on added urgency since late 2008 as concerns over the health of leader Kim Jong-il have intensified.

Kim, who suffered an apparent stroke in 2008, may die within three years, South Korean media have reported. His death is thought to have the potential to trigger instability and a power struggle in the North.

Gen. Walter Sharp, the top U.S. commander in South Korea, said the possibility of turmoil in the North is of real concern, citing the country's economic weakness, malnourishment in both the military and general population, and its nuclear weapons.

"The possibility of a sudden leadership change in the North could be destabilizing and unpredictable," he said in testimony beforethe U.S. House of Representatives appropriations committee this week.