Grim-faced soldiers trudged through broken glass and blackened debris Thursday on a remote South Korean fishing and garrison island that became a war zone this week.

The calm of the craggy outpost of Yeonpyeong was shattered Tuesday when North Korea unleashed a barrage of rockets and shells that killed four people and sent most of its terrified residents fleeing for the mainland.

Two days on, lost dogs roamed through charred ruins in search of food, shattered glass sparkled on the eerily deserted streets, and an abandoned children's pram bore testimony to the hasty departure of one panicked family.

The atmosphere was tense, with rifle-toting troops from the frontline island's military bases and coastguard officers patrolling streets, while a small flotilla of patrol boats was anchored in choppy waters.

Half a dozen marines were bracing against the piercing cold wind, boiling water on a small fire of tree branches at a coastal military base from where, on a clear day, they can see the coast of North Korea.

Company commander Captain Kim Jung-Soo recalled how the troops were carrying out the exercise on Tuesday which North Korea says provoked its attack, despite the fact the South Koreans fired south, away from North Korea.

"We were doing firing drills here until 2:30 pm, and right after then artillery shells started flying this way," the captain said, pointing at their anti-aircraft gun which was badly mangled in the blast.

Two marines still wore helmets that were scorched in the explosion.

The attacks -- the first shelling of civilians since the 1950-53 Korean War -- also hit administrative offices, a police station and several homes, knocked out electricity to half the population and started forest fires.

"Thank God the shells didn't reach the pre-school and elementary school," where children were studying at the time, said coastguard Jo Young-Hyun.

In the island's village, lines of soldiers passed along boxes of bottled water, candy and fruits being unloaded from boats as messages bellowing from loudspeaker urged the remaining residents to leave the island.

Most of the island's almost 2,000 inhabitants have already fled, leaving behind day-to-day items in their haste, such as clothes on washing lines and vegetables waiting to be fermented into kimchi, the national delicacy.

But Shin Yoo-Taek, 71, who was born on the island, said he was staying.

He said when the attack came he had just gone to the marine barracks to pick up leftover food for his dozens of pigs.

"Then suddenly fire rained down all over the place," he said. "I thought we were in the middle of brush fires until a loudspeaker told us of the attack.

"All my family -- apart from my son, who is a firefighter, and myself -- have left the island."

But he said he couldn't leave: "I keep scores of pigs here. I cannot leave them here. I'll rather die here with them."

Noh Young-Chul, a 60-year-old, was also staying despite fears of another bombardment when US and South Korean forces carry out a joint four-day military exercise in the Yellow Sea that starts on Sunday.

"I am worried that the North might attack us again," he told AFP. "But I won't leave here."

Park Sun-Bi, 85, a long-time resident, was about to join the exodus with her son, who had taken the ferry on Thursday to come and get her.

"I'm just going to pack some things I need. Why stay here? There's nobody here now. They're all gone," she said. "I wonder when I will ever return."