Japan's Kyodo news agency, quoting a government official, had earlier said up to four missiles had been fired. But Japanese officials later said three missiles were launched.
North Korea's missile program is based on Scud technology provided by the former Soviet Union or Egypt, according to American and South Korean officials. North Korea started its Rodong-1 missile project in the late 1980s and test-fired the missile for the first time in 1993.
North Korea had observed a moratorium on long-range missile launches since 1999. It shocked the world in 1998 by firing a Taepodong missile over Japan and into the Pacific Ocean.
On Monday, the North's main news agency quoted an unidentified newspaper analyst as saying Pyongyang was prepared to answer a U.S. military attack with "a relentless annihilating strike and a nuclear war."
The Bush administration responded by saying while it had no intention of attacking, it was determined to protect the United States if North Korea launched a long-range missile.
On Monday, Undersecretary of State Nicholas Burns warned North Korea against firing the missile and urged the communist country to return to six-nation talks on its nuclear program.
The six-party talks, suspended by North Korea, involved negotiations by the United States, China, South Korea, Japan and Russia with Pyongyang over the country's nuclear program.
The United States and its allies South Korea and Japan have taken quick steps over the past week to strengthen their missile defenses. Washington and Tokyo are working on a joint missile-defense shield, and South Korea is considering the purchase of American SM-2 defensive missiles for its destroyers.
The U.S. and North Korea have been in a standoff over Pyongyang's nuclear weapons program since 2002. The North claims to have produced nuclear weapons, but that claim has not been publicly verified by outside analysts.
While public information on North Korea's military capabilities is murky, experts doubt that the regime has managed to develop a nuclear warhead small enough to mount on its long-range missiles.
Nonetheless, Lt. Gen. Michael Maples, director of the Defense Intelligence Agency, told U.S. lawmakers last week that officials took the potential launch reports seriously and were looking at the full range of capabilities possessed by North Korea.