FACTBOX-The disputed Korean Northern Limit Line and Yeonpyeong island

North Korea fired more than 200 artillery rounds into the sea near a tense maritime border with South Korea on Friday, with residents of two South Korean islands told to take shelter due to an unspecified "situation." Here are some facts about the disputed Northern Limit Line (NLL), the de facto maritime border between the two Koreas, and the island of Yeonpyeong which sits just south of the border:

NORTHERN LIMIT LINE (NLL) * The U.S. commander at the end of the 1950-53 Korean War drew up a sea border weeks after the Armistice was signed to prevent naval clashes off the peninsula's west and east coasts. The truce made no direct mention of sea borders.

* The NLL off the west coast is drawn around islands that lie well north of what would be the extension of the land border that had been part of the South's territory before the war. * The North made no issue of the sea NLL until 1973, when it began violating the limit and disputing its validity. An agreement signed in 1991 reaffirming non-aggression and recognising each other's political sovereignty appeared to settle the maritime dispute.

* Later in the 1990s, the North again began disputing the NLL, claiming the real border should lie far to the south. In 1999, a North Korean patrol boat violated the NLL by up to 10 km (6 miles) but retreated when a gunfight killed several North Koreans. * A skirmish near the same area three years later killed six South Korean seamen. In November 2009, South Korean navy ships responding quickly to an intruding North Korean vessel pounded it with thousands of rounds of gunfire, disabling it and killing at least one sailor.

* In 2010, a South Korean warship, the Cheonan, was torpedoed in the area, killing 46 sailors. A international team of investigators said the North was responsible, but Pyongyang denies involvement. YEONPYEONG ISLAND

* Yeonpyeong is a small island, home to just over 2,000 residents, 120 km (75 miles) west of Seoul. * The North Korea shoreline looms 11 km (7 miles) away from the cliffs at the north end of the island.

* Through binoculars it is possible to see the holes in the rock carved out by the North to house its artillery trained on the South. * South Korea maintains a military unit on the island.

* Locals on the island have lived with the North Korean threat for decades and grown used to it, rubbing elbows with South Korean troops on the daily, 2-1/2 hour ferry between the mainland and the island's port. * The local Coast Guard office shows clearly where fishing vessels are restricted to an area to the east and west of the island and not near the NLL, which is 1.6 km (one mile) away.

* Some 4,000 residents of Baengnyeong, a larger South Korean island further to the northwest, were also told to seek shelter.

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