The first was fired about 6:10 a.m. local time (5:40 p.m. Wednesday ET).
The military cannot confirm exactly when the missile exploded but said it "crashed shortly after it was launched," a Defense Ministry official said.
A second Musudan missile was fired Thursday at 6:56 p.m. local time (6:26 a.m. ET), the official said.
This launch, from the port city of Wonsan, is also believed to have failed, the official said.
The missiles went an estimated 200 meters off the launchpad, U.S. military officials said.
U.S. Strategic Command said its systems detected and tracked what were assessed as attempted North Korean missile launches, and determined they did not pose a threat to North America.
These were the second and third ground-based Musudan launches that have failed
in two weeks.
The United States tracked the previous attempt, which took place on April 15, the birthday of North Korean founder Kim Il Sung, but there was "no evidence the missile reached flight," a U.S. official told CNN's Barbara Starr.
The U.N. Security Council discussed the latest North Korean launches behind closed doors in New York.
Latest in line of tests
An increasingly belligerent North Korea continues to advance its nuclear weapons program in open defiance of international condemnation, including that of its closest regional ally, China.
Along with a purportedly successful nuclear test in January and a launch of what the North insisted was a rocket designed to put satellites in orbit, the reclusive regime scored a "partial success" as it fired a submarine-launched ballistic missile
over the weekend.
At the time, President Barack Obama said it was clear "that North Korea continues to engage in continuous provocative behavior. They have been actively pursuing a nuclear program, an ability to launch a nuclear weapon."
"Although more often than not they fail in these tests, they gain knowledge each time they engage in these tests," he added. "And we take it very seriously."
The failed launch comes only days after Pyongyang announced the opening date of the Workers' Party of Korea's Seventh Congress
, a rare and potentially significant gathering. The congress will open on May 6
and is the first such meeting since 1980.