Ann Ahe-ja, one of hundreds of exhausted evacuees from Yeonpyeong island arriving in the port of Incheon on a rescue ship, said Tuesday’s artillery barrage that killed four people — two of them civilians — had caught her by surprise.

“Over my head, a pine tree was broken and burning,” Ann told AP Television News. “So I thought ‘Oh, this is not another exercise. It is a war.’ I decided to run. And I did.”

U.N. Secretary-General Ban Ki-moon called the shelling of the island near the two nations’ disputed maritime border one of the “gravest incidents” since the Korean War.

South Korean troops remained on high alert. In Washington, President Barack Obama pledged to “stand shoulder to shoulder” with Seoul and called upon China to restrain its ally, North Korea.

The U.S. has more than 28,000 troops in South Korea to guard against North Korean aggression, a legacy of the bitter three-year conflict that ended in a truce, not a peace treaty, in 1953.

Seoul and Washington reaffirmed plans to hold joint military exercises later this week in the Yellow Sea, just 70 miles (110 kilometers) south of Yeonpyeong. The White House said the aircraft carrier USS George Washington would take part.

About 10 homes suffered direct hits and 30 were destroyed in the midafternoon barrage, according to a local official who spoke by telephone from the island just seven miles (11 kilometers) from the North Korean shore. About 1,700 civilians live on Yeonpyeong alongside South Korean troops stationed there.

“I heard the sound of artillery, and I felt that something was flying over my head,” said Lim Jung-eun, 36, who fled the island with three children, including a 9-month-old strapped to her back. “Then the mountain caught on fire.”

Many of those evacuated from Yeonpyeong had spent the night in underground shelters and embraced tearful family members on arrival in Incheon.

The shower of artillery from North Korea was the first to strike a civilian population. In addition to the two marines killed, the bodies of two men, believed in their 60s, were pulled from a destroyed construction site, the coast guard said. At least 18 people — most of them troops — were injured.

Officials in Seoul said there could be considerable North Korean casualties. North Korea’s main Rodong Sinmun newspaper published a military statement accusing South Korea of triggering the exchange, but did not mention any casualties.

The skirmish began after North Korea warned the South to stop carrying out military drills near their sea border, South Korean officials said.

When Seoul refused and fired artillery into disputed waters — away from the North Korean shore — the North retaliated by shelling Yeonpyeong.

Seoul responded by unleashing its own barrage of howitzers and scrambling its fighter jets.

North Korea, laying out its version of events, said the army warned the South several times that firing “a single shell” in its waters would draw a “prompt retaliatory strike.” A military official phoned a South Korean counterpart at 8 a.m. to urge Seoul to cancel the drills, the North’s news agency KCNA reported.

But the South Koreans — displaying their “crafty and vicious nature” — went ahead and fired dozens of shells some five hours later, prompting a defensive response, the report said.

Island resident Cha Tae-jung said a shell fell just 50 yards (meters) behind him.

“I was going to turn around, but I didn’t because I was reaching out for something in front of me instead, and at that moment, the bomb dropped,” Cha told YTN television.

“I think I’m alive because I did not turn around,” he told the South Korean broadcaster, his voice turning emotional. “Words cannot describe what happened.”

The Obama administration urged China to press North Korea to halt provocative action, saying Beijing has a duty to tell Pyongyang that deliberate acts “specifically intended to inflame tensions in the region” are not acceptable.

China said late Wednesday that it was “highly concerned” about the artillery exchange and urged restraint.

China “feels pain and regret about an incident causing deaths and property losses and is worried about the developments,” Foreign Ministry spokesman Hong Lei said in a statement. “We have always maintained that the relevant parties should, through dialogue and consultation, resolve disputes by peaceful means.”

Diplomats for countries on the U.N. Security Council said there had been no request for the 15-member council to hold a full, formal meeting about the shelling, but said some informal bilateral talks were being held among some members.

A spokesman for North Korea’s Foreign Ministry said Wednesday night that troops acted defensively in response to “extremely reckless military provocation,” the North’s official Korean Central News Agency said.

In Pyongyang, residents boasted that the exchange showed off their military’s strength and ability to counter South Korean aggression.

“I think this time our military demonstrated to the whole world that it doesn’t make empty talk,” Ri Pong Suk told TV news agency APTN in the North Korean capital.

Artillery and gunfire break out sporadically along the land and maritime borders dividing the two Koreas, and have brought deadly exchanges four times since 1999.

In March, North Korea was accused of sinking a South Korean warship, the Cheonan, killing 46 sailors. Pyongyang has denied responsibility.

The North’s most notorious act was the 1987 bombing of a South Korean airliner that claimed 115 lives. And in 1996, a group of North Korean spies slipped ashore from a submarine and killed three civilians and a South Korean army private.

South Koreans see Tuesday’s killing of civilians as taking the confrontation to a new level, one analyst said.

“It’s clearly a line for people, and crossing that line puts it in a different category,” said John Delury, an assistant professor at Seoul’s Yonsei University Graduate School of International Studies. “People here are feeling very conflicted, outrage and sorrow.”

On Yeonpyeong island, famous for its crabs, video from YTN showed burned-out buildings still smoldering and huge craters from the shelling,

One of the main targets was a supermarket that once housed the office of a military intelligence agency, local official Choi Chul-young told The Associated Press by telephone from the island.

Chung Doo-sun, who lives in the nearby city of Gimpo, said his daughter lives on the island and “was crying and told me the windows of her home were all shattered.”

Seoul, the South Korean capital of more than 10 million people, was calm, although the skirmish weighed on people’s minds.

“I never felt anxious in the past, even after the Cheonan warship incident,” said Lee Ho-chul, 30. “But it feels different this time since civilians were hurt.”

South Korea said it would strengthen military forces in the western waters near Yeonpyeong and halt shipments of humanitarian aid to the North.

At a military hospital in Seongnam, just outside Seoul, relatives wailed as they filed out of a memorial for the two dead marines.

“Bring him back!” cried out Kim O-bok, mother of 22-year-old marine Seo Jeong-woo, as she collapsed.

The deadly exchange of fire came just six weeks after North Korean leader Kim Jong Il’s youngest son and heir-apparent, Kim Jong Un, made his international public debut at a massive military parade.

It also came days after Pyongyang showed off its uranium enrichment facility to a visiting U.S. scientist, raising new concerns about its pursuit of atomic weapons.

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Kwang-tae Kim reported from Seoul. AP writers Ian Mader and Foster Klug in Seoul, Cara Anna in Beijing, Anita Snow at the United Nations, and Anne Gearan in Washington contributed to this report.