North Korea fired short-range missiles into international waters in the Yellow Sea as part of a routine military exercise, a South Korean defence ministry official said on Wednesday.
The missiles were fired on Tuesday, the official told AFP on condition of anonymity, adding the type and number were unclear.
"The latest missile launch was seen as part of routine military exercises," he said.
The firing came just days ahead of Friday's anniversary of the North's ruling Workers' Party.
Analysts are closely watching the event to see if the communist state's top leader, Kim Jong-Il, who reportedly suffered a stroke in August, will appear.
Chosun Ilbo newspaper, quoting a government source, said the missile launches aimed to "show that the military is still in good shape" despite rumours about Kim's health.
Seoul's Joint Chiefs of Staff office declined to say anything. "We don't comment on North Korean intelligence," a spokesman said.
Local media said the North fired either surface-to-ship KN-01 or KN-02 missiles or Russian-designed ship-to-ship Styx missiles.
The KN-01 is a modified version of the Chinese-made Silkworm with a range of 110 to 120 kilometres (68-74 miles). The Styx has a range of 46 to 50 kilometres.
Chosun, quoting intelligence sources, said Seoul had spotted the North's army preparing to fire KN-01 and Styx missiles from Chodo, an island navy base in North Hwanghae province.
It had declared an exclusion zone for ships in the area.
JoongAng Ilbo newspaper and Yonhap news agency said the North launched two missiles while Dong-A Ilbo said the North fired only one.
"It appears to have fired KN-02 or Styx missiles into the international sea from North Korean waters," Yonhap said.
The North has carried out short-range missile tests many times before. It fired three Styx off its west coast on May 30 and fired three or four missiles of the same type on March 28.
The latest exercise comes amid intense efforts to save a nuclear disarmament agreement, which is in danger of collapsing because of a dispute over verification of the North's declared nuclear programme.
Chief US nuclear negotiator Christopher Hill visited Pyongyang last week to try to rescue the six-nation deal.
Secretive North Korea is bridling at a US-inspired verification plan which reportedly calls for it to open up undeclared suspected nuclear facilities and to let inspectors take samples of material.
The North has also carried out long-range missile tests in recent years which have alarmed its neighbours.
It test-launched a Taepodong-1 over Japan from the Musudan-ri east coast site in 1998. A longer-range Taepodong-2 was launched from the same site in 2006 but failed.
Japan's defence ministry declined comment on the latest launches.
"We are always collecting information regarding North Korean missiles but we would like to decline to comment on individual cases," a spokesman said.
"At this stage, we do not believe there has been a serious problem affecting our country's security."