The United States and South Korea held a joint air exercise on Sunday in response to North Korea's latest missile test, according to the Defence Ministry in Seoul.
The ministry said the exercise in South Korea's air space included US B-1B long-range bombers and fighter jets from both countries.
The exercise can be seen as a direct reaction to Pyongyang's firing of an intercontinental ballistic missile (ICBM) on Saturday.
State news agency KCNA said that North Korea test-launched a Hwasong-15, a type of intercontinental ballistic missile (ICBM), from Pyongyang airport on Saturday afternoon in what was termed a "surprise ICBM launching drill."
KCNA said that the missile flew 989 kilometres and travelled for just over an hour "before accurately hitting the pre-set area in open waters of the East Sea of Korea." The statement used the North Korean name for the Sea of Japan.
UN resolutions prohibit North Korea from testing ballistic missiles of any range, which - depending on their design - can also be equipped with a nuclear warhead.
It would be the first test of such a missile in more than a month.
South Korea's Joint Chiefs of Staff (JCS) said on Saturday they detected the launch of a suspected long-range ballistic missile from the Sunan area in Pyongyang at 5:22 pm (0822 GMT) on Saturday, South Korea's Yonhap news agency reported.
The missile was launched at a high angle, initially reaching an altitude of up to 5,768 kilometres.
KCNA said that the Saturday launch was "actual proof" of North Korea's "strategic nuclear force's consistent efforts to turn its capacity of fatal nuclear counterattack on the hostile forces into the irresistible one as well as a guarantee for and a clear proof of the sure reliability of our powerful physical nuclear deterrent."
North Korea had threatened "unprecedented" military action on Friday over plans by the United States and South Korea to hold joint military drills in March.
South Korea's general staff accused its neighbouring country of a "serious provocation." The US also strongly condemned the missile test.
The situation on the Korean peninsula is currently very tense. Nuclear-armed North Korea has conducted a string of missile tests in the past few weeks.
Observers fear that Pyongyang's first nuclear test in years is imminent.
