North Korea's military declared a no-sail zone off its west coast, including waters around South Korea's border islands, in an indication that it is ready to fire missiles into the South's territorial waters.

The North's navy warned ships from sailing in designated waters along the maritime border, a move that preceded previous missile test-launches, defense officials in Seoul said Tuesday. The warning went into effect Monday and it to last until March 29.

The no-navigation zone covers an area 2.5 miles south of the Northern Limit Line, an inter-Korean maritime border demarcated by the United Nations after the 1950-53 Korean War. The zone is close to islands that belong to South Korea.

It is the first time that the North's no-sail zone included the South's waters, a move apparently aimed at boosting its territorial sovereignty.

"North Korea declared a shipping ban in the sea south of the NLL. The South Korean military is closely examining whether the move is part of the North's winter military exercise or related to its preparation to test-launch short-range missiles," a military official said.

"We are fully ready to defend the maritime demarcation line and will act sternly in case of North Korean provocations. No particular move from the North has been detected yet."

The poorly marked western sea border has long been a source of conflict between the two Koreas, as the North does not recognize the NLL border. The North insists on its own maritime border, far south of the NLL, which includes the South's five border islands.

The territorial contest triggered armed clashes in 1999 and 2002, when the two Koreas traded naval gunfire that left dozens of casualties on both sides.

A third naval skirmish took place last November when a warship from the North intruded into the South's waters. The 2-minute exchange of gunfire left the North's boat badly damaged, with no casualties on the South's side. Military analysts warned that the North, humiliated by the defeat, could stage a retaliatory attack against the South.

Last month North Korea threatened to fire shells into waters around South Korea's border islands, declaring waters along its disputed sea border a "firing zone" and warning ships from the South to stay away from the area.

Cho Sung-yurl, a researcher at the Institute for National Security Strategy in Seoul, said the North could attempt to occupy the South's border island of Baekryeong, which has a population of 4,500.

"The North could attempt to occupy the border island and then threaten to use its nuclear weapons to deter the South's countermeasures," he said.

Last week the North's military threatened to stage a "sacred retaliatory battle" against the South in response to Seoul's reported contingency plan to handle a potential regime collapse in Pyongyang. In a show of its readiness to translate the threat into real military action, the North conducted a joint drill by the army, navy and air force under direct guidance of its leader Kim Jong Il.

On Sunday, the North again threatened the South, saying it would take "prompt and decisive" military action against any South Korean attempt to violate the North's "dignity and sovereignty," and would blow up major targets in the South.

"Our revolutionary armed forces will regard the scenario for 'pre-emptive strike,' which the South Korean puppet authorities adopted as a 'state policy,' as an open declaration of war," the General Staff of the Korean People's Army said in a statement.

The warning came in response to South Korean Defense Minister Kim Tae-young's remarks that the South should launch a pre-emptive strike on the North if there was a clear indication the country was preparing a nuclear attack.

Military sources in Seoul said the North's series of tension-raising moves along the sea border are aimed at reminding the United States that the Korean Peninsula is technically in a state of war, as the 1950-53 armed conflict ended without a peace treaty, in a bid to back its longstanding demand of a peace treaty with Washington.

U.S.-led U.N. forces rescued South Korea, which was almost occupied by the North's surprise invasion on June 25, 1950. The war ended in July 1953, as the U.S.-led U.N. Command signed an armistice agreement with North Korea and China, whose 1 million troops supported North Korea.

Earlier this month, the North proposed talks for a formal peace treaty with the United States, saying a peace pact is "essential" to build confidence between the two sides and achieve denuclearization on the peninsula.