But where are these islands, who lives there, and what is life like on one of the front lines between North and South?

AFP takes a look at what we know:

Where are they?

Both islands are extremely close -- less than two kilometres (1.5 miles) -- to the de facto maritime border between the two Koreas known as the Northern Limit Line (NLL).

Control of the islands was awarded to South Korea in the 1953 armistice agreement, which ended fighting in the Korean War, but the deal did not specify an exact maritime border -- and was never replaced by an official peace treaty, meaning the countries remain technically at war.

Although they're controlled by Seoul and on the southern side of the de facto maritime border, the islands, Yeonpyeong and Baengnyeong, are much closer to North Korea.

Baengnyeong is some 210 kilometres west of Seoul, but just 14 kilometres from the North Korean mainland. Yeonpyeong is about 115 kilometres west of Seoul.

Baengnyeong island is of strategic importance for Seoul, as it helps determine control over vital Yellow Sea shipping lanes, without which South Korea's Incheon harbour would be cut off from the world.

Has this happened before?

In 2010, in response to a South Korean live-fire drill near the sea border, the North bombarded Yeonpyeong island killing four South Koreans -- two soldiers and two civilians.

That was the first attack on a civilian area since the 1950-53 Korean War.

The South returned fire, with the resulting exchange lasting more than an hour, as the two sides traded more than 200 shells, sparking brief fears of a full-fledged war.

During the 2010 incident, a North Korean artillery shell slammed into Kim Soon-ok's house and blew up the top floor.

The cafe owner told AFP in 2018 that the attack forced her to run barefoot into the street, terrified and screaming that war had broken out.

"Whenever I hear a loud thump, I check outside by reflex," Kim said at the time.

"I always go to bed with a bag packed just in case."

What about the buffer zone?

The de facto maritime border was the scene of deadly naval clashes in 1999, 2002 and 2009.

Baengnyeong has also long been a potential military flashpoint: North Korea's Kim Jong Un claimed in 2013 that he could "rain down a sea of fire" on the island, and then staged amphibious mock invasion drills in 2017.

In 2018, during a period of warmer ties, both sides signed a military deal aimed at reducing tensions, in which they committed to preventing accidental clashes in a so-called buffer zone along the NLL.

But Seoul partially suspended the agreement in November to protest Pyongyang's launch of a spy satellite, with the North jettisoning the entire deal soon after.

"It is not unusual for North Korea to fire artillery into the West Sea during its winter exercises," said Leif-Eric Easley, a professor at Ewha University in Seoul.

"What's different this year is the two Koreas have recently backed away from a military confidence-building agreement, and Kim Jong Un has publicly disavowed reconciliation and unification with the South.

"Pyongyang could miscalculate that its claimed nuclear weapons give it control over escalation in case of a deadly incident."

What are they like?

Every corner of Yeonpyeong holds a grim reminder of its proximity to North Korea and the deadly attack in 2010.

Seoul has stationed extra troops and weaponry on the island, with military posts scattered throughout, and the sounds of practice artillery fire frequently reverberate across the area.

Education sessions on emergency evacuation are a regular event, usually held at one of a handful of bunkers.

And at the centre of Yeonpyeong village, two houses shelled in 2010 have been preserved as an education centre and macabre tourist attraction, including rusty burned bikes and slabs of smashed concrete stairs.

But residents lament that the bombardment has scared away tourists since 2010.

Local media reports say the low-income population in Yeonpyeong has increased by more than four times since the deadly shelling 14 years ago.

Tourist destination?

Despite the dangers -- the ferry to the island takes a roundabout route to make it less vulnerable to North Korean attack -- Baengnyeong is a niche tourist destination.

Hundreds of mostly domestic tourists visit each week, drawn by both the proximity to the North, and also the natural beauty of the location.

The island's ancient geological features hold a lesson for those worried about its frontline position, Kim Chang-hee, 75, a park guide, told AFP in 2022.

"This island is over 10 million years old," she said. "Korea has only been divided for 70 years. One day, this too will be a distant memory."