A South Korean patrol boat fires during a drill off the western coast town of Taean on May 27, 2010. South Korea's navy staged a major anti-submarine exercise, its first show of strength since tensions with North Korea flared over the sinking of one of Seoul's warships.

US and South Korean soldiers salute during a ceremony to commemorate US Memorial Day at a US Army base in Seoul on May 27, 2010. The top US commander in South Korea warned that North Korea should stop 'all acts of provocation' and pledged to support the South's measures to punish the North for sinking one of its warships.

Visitors look at a map of border lines between the two Koreas at the Korean War exhibition in Seoul, South Korea, Thursday, May 27, 2010. North Korea announced Thursday that it will scrap an accord aimed at preventing accidental naval clashes with South Korea in retaliation for Seoul blaming Pyongyang for a torpedo attack that sank a South Korean warship. (AP)

Honour Guards of the United Nations Command hold flags during a ceremony to commemorate the US Memorial Day at a US Army base in Seoul on May 27, 2010.

U.S. Army Gen. Walter L. Sharp, commander of U.S. Forces Korea, salutes after he offered flowers on the Books of Remembrance which lists the 33,870 American who died in Korean War, during the Memorial Day Honor Guard Ceremony at Knight Field, a U.S. military base in Seoul, South Korea, Thursday, May 27, 2010. (AP)

US soldiers march during a ceremony to commemorate the US Memorial Day at a US Army base in Seoul on May 27, 2010.

SEOUL, SOUTH KOREA - Commander of the United Nations Comand (UNC), Combined Forces Command (CFC), and United States Forces Korea (USFK) Gen. Walter Sharp saltues during a ceremony commemorating U.S. Memorial Day at Yongsan U.S. military base on May 27, 2010 in Seoul, South Korea. Memorial Day honours American men and women who have died in military service.

SEOUL, SOUTH KOREA - Honour Guards of the United Nations Command (UNC), the Combined Forces Command (CFC) and the United States Forces Korea (USFK) parade during a ceremony commemorating U.S. Memorial Day at Yongsan U.S. military base on May 27, 2010 in Seoul, South Korea. Memorial Day honours American men and women who have died in military service.

A South Korean patrol boat drops a depth charge during a drill off the western coastal town of Taean May 27, 2010. South Korea's navy staged anti-submarine drills on Thursday in tense waters bordering North Korea amid signs that China, under pressure from regional powers, is reviewing ties with the isolated communist state.