INCHEON, South Korea/WASHINGTON (Reuters) - The United States said on Wednesday it believed North Korea’s shelling of a South Korean island this week was an isolated act tied to leadership changes in Pyongyang and called on China to use its influence to stop the North’s provocative behavior.

Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said the United States was working with allies on ways to respond but that “It’s very important for China to lead.”

“The one country that has influence in Pyongyang is China and so their leadership is absolutely critical,” Mullen told a U.S. television talk show.

A day after North Korea rained artillery shells at the island of Yeonpyeong, killing two civilians, a U.S. aircraft carrier group set off for Korean waters on Wednesday to take part in drills.

Although the U.S. Forces Korea said the exercise had been planned well before the attack, many thought the move would enrage the North and unsettle its ally, China.

State Department spokesman P.J. Crowley also said the United States expects China to use its influence to get North Korea to cease its provocative behavior, saying Beijing could play a “pivotal” role in helping to calm the situation.

Mullen said he believed the attack was linked to the succession of the reclusive state’s leadership.

Widely thought to be in failing health, Kim Jong-il appointed his younger son to key posts in September, a move seen as grooming him to be the North’s next leader. But Kim Jong-un, has no real support base, and with the economy in dire straits there is a risk powerful military or government figures may decide the time is opportune for a power grab.

Tuesday’s attack by the North was the heaviest since the Korean War ended in 1953 and marked the first civilian deaths in an assault since the bombing of a South Korean airliner in 1987.

It was one of a series of provocations by Pyongyang in recent years, which have included two nuclear tests, several missile tests, and the sinking of a South Korean warship in March that killed 46 sailors.

North Korea said the shelling was in self-defense after Seoul fired shells into its waters near the disputed maritime border. The North’s KCNA news agency said the South was driving the peninsula to the “brink of war” with “reckless military provocation” and by postponing humanitarian aid.

“The DPRK that sets store by the peace and stability of the Korean peninsula is now exercising superhuman self-control, but the artillery pieces of the army of the DPRK, the defender of justice, remain ready to fire,” the agency said, referring to North Korea by its official name, the Democratic People’s Republic of Korea.

The nuclear-powered USS George Washington, which carries 75 warplanes and has a crew of over 6,000, left a naval base south of Tokyo and would join exercises with South Korea from Sunday to the following Wednesday, U.S. officials in Seoul said.

“An aircraft carrier is the most visible sign of power projection there is ... you could see this as a form of pre-emptive deterrence,” said Lee Chung-min of Yonsei University in Seoul.

Tuesday’s bombardment nagged at global markets, already unsettled by worries over Ireland’s debt problem and looking to invest in less risky assets. But by close of business on Wednesday, South Korea’s markets had recovered most of lost ground from the previous day.

SEOUL UNDER PRESSURE

The government in Seoul came under pressure for the military’s slow response to the provocation, echoing similar complaints made when a warship was sunk in March in the same area, killing 46 sailors.

Slideshow ( 34 images )

Defense Minister Kim Tae-young was grilled by lawmakers who said the government should have taken quicker and stronger retaliatory measures against the North’s provocation.

“I am sorry that the government has not carried out ruthless bombing through jet fighters during the North’s second round of shelling,” said Kim Jang-soo, a lawmaker of ruling Grand National Party and a former defense minister.

Prior to the public comments from Washington, China’s Foreign Ministry had urged the two Koreas to show “calm and restraint” and engage in talks as quickly as possible to avoid an escalation of tensions.

“China takes this incident very seriously, and expresses pain and regret at the loss of life and property, and we feel anxious about developments,” said spokesman Hong Lei.

Slideshow ( 34 images )

China has long propped up the Pyongyang leadership, worried that a collapse of the North could bring instability to its own borders and also wary of a unified Korea that would be dominated by the United States, the key ally of the South.

But Beijing has said previously that it sees as a threat to its security and to regional stability any joint U.S.-South Korea exercises in the waters between the Korean peninsula and China.

“China will not welcome the U.S. aircraft carrier joining the exercises, because that kind of move can escalate tensions and not relieve them,” said Xu Guangyu, a retired major-general in the People’s Liberation Army who now works for a government-run arms control organization.

SEOUL CALM

Seoul, a city of over 10 million, was bustling as normal on Wednesday, a sunny autumn day, although developments were being closely watched by office workers on TV and in newspapers.

“My house was burned to the ground,” said Cho Soon-ae, 47, who was among 170 or so evacuated from Yeonpyeong on Wednesday.

“We’ve lost everything. I don’t even have extra underwear,” she said weeping, holding on to her sixth-grade daughter, as she landed at Incheon.

South Korea, its armed forces technically superior though about half the size of the North’s one-million-plus army, warned of “massive retaliation” if its neighbor attacked again.

But it was careful to avoid any immediate threat of retaliation, which might spark an escalation of fighting across the Cold War’s last frontier.