Pyongyang's flurry of launches on Wednesday and Thursday included an intercontinental ballistic missile and one that landed near South Korea's territorial waters for the first time since the end of the Korean War in 1953.

The United States and South Korea have warned that the series of launches could culminate in a nuclear test by North Korea, and extended their largest-ever air force drills to Saturday in response.

A South Korean defence ministry official told AFP that a US Air Force B-1B strategic bomber would participate on the last day of the exercises, dubbed Vigilant Storm, which were originally scheduled from Monday to Friday this week.

"B-1B is scheduled to participate in the afternoon training," the official said, without providing further details.

The show of force came a day after South Korea scrambled fighter jets in response to what it said was the mobilisation of 180 North Korean warplanes.

Pyongyang has ramped up its missile launches in protest over the US-South Korea air drills. Such exercises have long infuriated North Korea, which sees them as rehearsals for an invasion.

Pyongyang had called Vigilant Storm "an aggressive and provocative military drill targeting" North Korea, and said the United States and South Korea would "pay the most horrible price in history" if they continued.

North Korea is particularly sensitive about these drills, experts say, as its air force is one of the weakest links in its military, lacking high-tech jets and properly trained pilots.

Pyongyang has been particularly angered in the past by the deployment of US strategic weapons such as B-1Bs and aircraft carrier strike groups, which have been deployed to and near the Korean peninsula in times of high tension.

While the B-1B no longer carries nuclear weapons, it is described by the US Air Force as "the backbone of America's long-range bomber force" that can strike anywhere in the world.

The United States on Friday assailed China and Russia at the UN Security Council for having "enabled" North Korea.

US Ambassador Linda Thomas-Greenfield dismissed criticism of the drills with South Korea as North Korean "propaganda" and said they posed no threat to other countries.