A day earlier, the North expelled Seoul officials from a joint industrial complex north of the border, after South Korea told its neighbor to improve human rights and stop delaying nuclear disarmament talks if it wants to receive aid to keep its economy afloat, Reuters reported.
South Korea downplayed the launches, saying they were part of the Communist state's routine military training. It did not give details about the type and number of missiles fired by the North, but the South's national Yonhap News Agency quoted government sources as saying that the North launched three to six missiles.
"We believe the North does not want a deterioration of relations between the South and the North," South Korean presidential spokesman Lee Dong-kwan said.
Pyongyang was basically sending two messages with the launch, Keio University Korea expert Masao Okonogi told Reuters from Tokyo.
One was aimed at the United States after talks in Geneva, showing the North's dissatisfaction with Washington's pressure to come clean on uranium enrichment and ties with Syria, he said. The other was a riposte to the Lee government's shift in stance.
"They are warning Seoul not to go back on things agreed between the North and the South," Okonogi said.
North Korea has more than 1,000 missiles, at least 800 of them ballistic, that can hit all of South Korea and most of Japan, experts have said. Its launches are often timed to coincide with periods of political tension.
Hours after the launchings, Pyongyang further rattled nerves in Seoul when it accused South Korean warships of violating the disputed western sea border and warned of possible naval clashes along the world's most heavily armed frontier, The New York Times reported.
"The South Korean warmongers' reckless provocations are creating a situation throughout the western maritime border in which armed clashes can erupt any time," the North's navy command said in a statement carried by the country's official Korean Central News Agency. "They should not misjudge our patience and restraint."
North Korea's navy command said the South "infiltrated 14 warships" into Communist waters on Wednesday alone. The South's Defense Ministry immediately denied the accusation, but experts warned that chances were growing that North Korea would attempt a naval skirmish on the volatile sea border.
North Korea's first and only nuclear test in 2006 created more international concern about its missile capabilities. But it is unknown whether the North has the technology to tip its missiles with nuclear warheads.
Experts said the North was seeking to improve its bargaining leverage by escalating tensions at a time when negotiations with Washington over North Korea's nuclear program were not proceeding in its favor and South Korea was becoming less generous with economic aid for the North.
"If the latest gestures follow form, all the great powers with a stake in the Korean Peninsula will have their negotiators scurrying from capital to capital, armed with statements, giving press conferences and holding meetings until calm returns in a fiery display of demands, recriminations and good old-fashioned rhetoric," Donald Kirk wrote in the Asia Times. "That, at any rate, is the optimistic scenario."