Factbox: What is the Korean Northern Limit Line?

(Reuters) - North Korea Tuesday fired dozens of artillery shells at a South Korean island near a disputed western sea border, killing two soldiers and setting houses ablaze.

The exchange of fire between the two Koreas was centered around Yeonpyeong island, just 120 km (75 miles) west of Seoul, and the near the contested Northern Limit Line (NLL).

The South said it was conducting a test firing exercise in the area before North Korea fired on the island.

Here are some facts about the 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, as the maritime border is called, 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 recognizing each other’s political sovereignty appeared to settle the maritime dispute.

* 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 as much as 10 km (6 miles) but went home defeated when a gun fight 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 March, 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.