South Korean and US officials said the first rocket failed over the Sea of Japan
North Korea has launched two mid-range missiles off its eastern coast, even though such attempts are banned by the UN.
South Korean and US officials said the first rocket failed over the Sea of Japan, while a second missile was test-fired hours later, and reportedly flew for 400km before it fell.
Pyongyang has now unsuccessfully attempted to launch six Musudan missiles, which could be capable of striking targets across South Korea and Japan, as well as US military bases as far away as Guam.
The rockets have a range of anywhere between 1,550 and 2,500 miles.
Japanese politicians have described the tests as a "grave provocative action", with the country's prime minister warning such ballistic missile launches "clearly cannot be tolerated".
The US has stressed the missiles never posed a threat to North America, but a State Department spokesman condemned the launches.
Meanwhile, South Korea's presidential office is holding a security meeting to discuss the launches.
Defence officials in Seoul say Pyongyang's previous attempts have resulted in the missiles crashing or exploding in mid-air shortly after launch.
North Korea is banned from using any type of ballistic missile technology, but has regularly fired short-range rockets from its eastern coast.
The isolated state has been trying to intimidate neighbours with its ongoing nuclear weapons programme, which is said to have the ultimate aim of developing a missile that could reach the US.
Intelligence suggests North Korea has more than 30 Musudan missiles in its arsenal, yet it hasn't attempted to test-fire them until this year.
In January, the White House cast doubt on North Korea's claim to have conducted its first "successful" test of a miniature hydrogen bomb.
Pyongyang said the detonation was a "historic" event, but White House press secretary Josh Earnest said US analysis was "not consistent" with the claim.