The bombers flew further north of the Demilitarised Zone that divides the Korean Peninsula than any US military aircraft in the 21st Century.

In the days after the flight, North Korea has been moving airplanes and boosting defences on its east coast, South Korea's Yonhap News Agency reported on Tuesday.

China's UN Ambassador Liu Jieyi voiced alarm at the escalating rhetoric between North Korea and the United States.

"We want things to calm down. It's getting too dangerous and it's in nobody's interest," he told Reuters.

"We certainly hope that they will see that there is no other way than negotiations to solve the nuclear issue on the Korean Peninsula

"The alternative is a disaster."