JIM LEHRER: There was artillery fire over South Korea today. Two South Korean marines were killed. It was a stark reminder that North and South Korea are still technically at war 57 years after their Korean War armistice.
We begin our coverage with a report narrated by Lindsey Hilsum of Independent Television News.
LINDSEY HILSUM: Closed-circuit TV recorded the moment North Korean shells hit Yeonpyeong Island. South Korean television broadcast the images of an incident which demonstrates just how fragile is the truce between the two Koreas.
The North says the South fired first. The South says its shots were merely part of a scheduled military exercise, and this was a provocation which endangered the civilians who live on the island.
LEE CHUN-OK, resident, Yeonpyeong Island (through translator): I thought the South Korean military was carrying out a heavy artillery exercise, since it had already been announced. When the bomb went off near my house, which collapsed, I realized it was North Korea who had fired. I thought I would die if I left the house, so I stayed put.
LINDSEY HILSUM: Loudspeakers sounded across all the islands in the region, telling residents to run to bomb shelters.
SHIN SUNG-HEE, resident, Yeonpyeong Island (through translator): The shells didn't hit just one place. There were fires everywhere. And it was chaos.
LINDSEY HILSUM: The three civilians and 16 military personnel who had been injured were airlifted to hospital near the South Korean capital.
The president visited the joint chiefs of staff and defended South Korea's decision to fire back, despite the risk of escalation.
LEE MYUNG-BAK, South Korean President (through translator): Hundreds of statements and meetings wouldn't work. I think it's a duty for the army to respond with action. We can never tolerate unconditional attacks against civilians.
LINDSEY HILSUM: From Pyongyang, just one statement, saying South Korea started it.
WOMAN (through translator): The Revolutionary Armed Forces of the Democratic People's Republic of Korea, standing guard over the inviolable territorial waters of the country, took the decisive military step of reacting to the provocation of the puppet group with a prompt, powerful physical strike.
LINDSEY HILSUM: The North disputes the maritime border, contending its territorial waters continue further south.
In March, the South Korean ship Cheonan was sunk, killing 46 sailors. An international investigation said the North Koreans had torpedoed the vessel. In August, North Korea fired artillery shells into the Yellow Sea, after the South Korean navy had finished live-fire exercises in the area.
And, just last month, shots were fired across the DMZ, near Cheorwon, one day after the North Koreans had denounced the South for refusing to hold more military talks.
President Kim Jong Il recently introduced his son and heir, Kim Jong-un, to an obedient nation. Such is the secrecy of the regime, we don't know if Kim the younger, now a general, is involved in this escalation. Last week, a U.S. scientist visiting the Yongbyon site reported that he had seen more than 1,000 shiny new centrifuges, evidence that the North's nuclear program is well-advanced.
SIEGFRIED HECKER, Stanford University: By having me there, they could actually attest to the fact that it was a light-water reactor, which is much more suitable for making electricity than for bombs. But I think, to some extent, they also wanted to demonstrate to us, look, we have the technology and we know how to do this, because what I saw when I walked in that facility was just amazing.
They already have the bomb. And they have the plutonium bomb. So, if they make a lot of highly enriched uranium, then, all of a sudden, they could have an arsenal of dozens of bombs.
LINDSEY HILSUM: This is the kind of incident which fills diplomats with fear. The Russian foreign minister called it a colossal danger. The Chinese spokesman said nuclear talks must restart. And President Obama's envoy visiting China firmly blamed North Korea.
STEPHEN BOSWORTH, U.S. Special Representative for North Korea Policy: The U.S. strongly condemns this aggression on the part of North Korea. And we stand firmly with our allies.
LINDSEY HILSUM: As darkness fell fires were still burning on Yeonpyeong Island.