North Korea warned on Friday that US-South Korean plans for military maneuvers put the peninsula on the brink of war, and appeared to launch its own artillery drills within earshot of an island it showered with a deadly barrage this week.

The fresh artillery blasts came just after the top US commander in South Korea, Gen. Walter Sharp, toured the country's Yeonpyeong Island in a show of solidarity with Seoul and to survey damage from Tuesday's hail of North Korean artillery fire that killed four people.

An official at the South Korean Joint Chiefs of Staff said about 20 blasts were heard on Yeongpyeong coming from North Korea's mainland some 7 miles (11 kilometers) to the north, and that nothing landed on South Korean territory. The military official spoke on condition of anonymity, citing military policy.

Tensions have soared between the Koreas since the North's strike on Tuesday destroyed large parts of this island, killing two civilians as well as two marines in a major escalation of their sporadic skirmishes along the sea border.

The attack — eight months after a torpedo sank a South Korean warship in nearby waters, killing 46 sailors — has also laid bare weaknesses in South Korea's defense 60 years after the Korean War. The incident forced South Korea's beleaguered defense minister to resign on Thursday.

The heightened animosity between the Koreas is taking place as the North undergoes a delicate transition of power from leader Kim Jong Il to his young, inexperienced son Kim Jong Un, who is in his late 20s and is expected to eventually succeed his ailing father.

As Washington and Seoul pressed China to use its influence on Pyongyang to ease tensions amid concerns of all-out war, the US prepared to send a nuclear-powered aircraft carrier to South Korean waters for joint military drills starting Sunday.

The North, which sees the drills as a major military provocation, unleashed its anger over the planned exercises in a dispatch earlier on Friday. "The situation on the Korean peninsula is inching closer to the brink of war," the report in the North's official Korean Central News Agency said.

A North Korean official boasted that Pyongyang's military "precisely aimed and hit the enemy artillery base" as punishment for South Korean military drills, and warned of another "shower of dreadful fire," KCNA said.

The regime does not recognize the maritime border drawn by the UN in 1953, and considers the waters around Yeonpyeong Island its territory.

Yeonpyeong Island, home to South Korean military bases as well as a civilian population of about 1,300 people, lies only 7 miles (11 kilometers) from North Korean shores and is not far from the spot where the Cheonan sank in an explosion in March.

Gen Sharp called Tuesday's attack on Yeonpyeong a clear violation of an armistice signed in 1953 at the end of the three-year Korean War. "We at United Nations Command will investigate this completely and call on North Korea to stop any future attacks," he said during a visit to the island on Friday.

Washington keeps more than 28,000 troops in South Korea to protect the ally from aggression — a legacy of the Korean War that is a sore point for North Korea, which cites the US presence as the main reason behind its need for nuclear weapons.

Dressed in a heavy camouflage jacket, army fatigues and a black beret, he walked down a heavily damaged street strewn with debris from buildings. Around him were charred bicycles and shattered bottles of soju, Korean rice liquor.

Hours later, after Sharp had returned to Seoul, AP photographers at an observation point on the northwest side of Yeonpyeong heard four explosions and saw at least one flash of light on the North Korean mainland.

There were no immediate reports of damage. Only a few dozen residents remain on Yeonpyeong, with most of the population of 1,300 fleeing in the hours and days after the attack as authorities urged them to evacuate.

On Thursday, President Lee Myung-bak ordered reinforcements for the 4,000 troops on Yeonpyeong and four other Yellow Sea islands, as well as top-level weaponry and upgraded rules of engagement. He also sacked Defense Minister Kim Tae-young amid intense criticism over lapses in the country's response to the attack.

Lee, dressed in a black suit, visited a military hospital in Seongnam near Seoul on Friday to pay his respects to the two marines killed in the North Korean attack.

Lee laid a white chrysanthemum, a traditional symbol of grief, on an altar, burned incense and bowed before framed photos of the two young men. Consoling sobbing family members, he vowed to build a stronger defense. "I will make sure that this precious sacrifice will lay the foundation for the strong security of the Republic of Korea," he wrote in a condolence book, according to his office.