The launches, which came two days after North Korea fired four short-range missiles, could further escalate tensions in the region as the U.S. tries to muster support for tough enforcement of the U.N. resolution imposed on the communist regime for its May nuclear test.
South Korea's Joint Chiefs of Staff said the missiles were fired Saturday morning, but declined to elaborate on the type. Two missiles were fired early Saturday, while a third was fired later in the morning, it said.
South Korea's Yonhap news agency quoted military officials as saying the missiles appeared to be a type of Scud missile.
A senior presidential official told The Associated Press the missiles fired are believed to have a range of less than about 300 miles (500 kilometers).
"Our military is fully ready to counter any North Korean threats and provocations based on strong South Korea-U.S. combined defense posture," the Joint Chiefs of Staff statement said.
North Korea's state news agency carried no reports on the launches.
The chief of U.S. Naval operations, Adm. Gary Roughead, said the American military was ready for any North Korean missile tests.
"Our ships and forces here are prepared for the tracking of the missiles and observing the activities that are going on," Roughead said after meeting Japanese military officials in Tokyo on Saturday.
Speculation had been high that the communist country might launch more missiles. North Korea had warned shipping to stay away from its east coast effective through July 10.
The senior presidential official cautioned that North Korea could fire more missiles in coming days, but said there was little possibility it could fire the intercontinental ballistic missile it threatened in April.
He spoke on condition of anonymity because he was not authorized to speak to media.
Saturday's launches came on July 4, or U.S. Independence Day. The North has a record of timing missile tests for the U.S. national day.
"The missiles were seen as part of military exercises, but North Korea also appeared to have sent a message to the U.S. through the missile launches," the presidential official said, without elaborating.
The Japanese government condemned the North's action.
The launch "is a serious act of provocation against the security of neighboring countries, including Japan, and is against the resolution of the U.N Security Council," Japanese Chief Cabinet Secretary Takeo Kawamura said in a statement. The statement also urged North Korea to suspend all missile development activities.
In Beijing, a Foreign Ministry spokesman said he had no immediate comment.
In 2006, Pyongyang fired a barrage of missiles, including a long-range Taepodong-2 that broke apart and fell into the ocean less than a minute after liftoff. Those launches while Americans were celebrating the Fourth of July holiday also came amid nuclear tensions with the U.S.
A long-range missile launch by North Korea toward the United States would directly flout a U.N. sanctions resolution punishing Pyongyang for its May 25 nuclear test.
Firing a ballistic missile on July 4 would be a snub to Washington, which has been trying to muster international support for a tough enforcement of the U.N. resolution that bans Pyongyang from any further nuclear or ballistic missile tests.
Despite early speculation fueled by Japanese media and the North Korean warning to shipping, spy satellites have apparently not detected any of the preparations that would normally presage a launch.