North Korea held a large anti-US rally in its capital city on Saturday, backing its leader Kim Jong Un as he exchanges insult-laden threats with President Donald Trump.

A huge crowd gathered in Pyongyang's Kim Il Sung Square, named for the current leader's grandfather and founder of North Korea, to rally against the US after Trump slammed North Korea's 'reckless behavior' last week.

Hours later, US bombers and escorts flew to the farthest point north of the border between North and South Korea by any such American aircraft this century in a show of military might, the Pentagon said.

During the rally, demonstrators in North Korea listened to speeches from senior officials excoriating the United States and its president parade of marchers carried signs with slogans such as 'decisive revenge' and 'death to the American imperialists'.

The crowd, including workers, officials and students, shouted phrases such as 'total destruction', according to the Korean Central News Agency, the state news service.

Later on Saturday night, US bombers and escorts flew into international airspace near North Korea to show how seriously Trump takes North Korea's missile tests, the Pentagon said in a statement.

'This mission is a demonstration of US resolve and a clear message that the president has many military options to defeat any threat,' the Defense Department spokesman Dana White said in a statement.

'North Korea's weapons program is a grave threat to the Asia-Pacific region and the entire international community,' it added. 'We are prepared to use the full range of military capabilities to defend the US homeland and our allies.'