U.S. President Donald Trump's recent attempt to look tough in his blistering war of words with North Korean leader Kim Jong Un has sowed divisions with Tokyo and Seoul — America's two key allies in Asia — risking a rift that could give the reclusive regime a strategic advantage, a former top White House official has warned.

In an interview with The Japan Times in Tokyo last week, Ben Rhodes, ex-deputy national security adviser to President Barack Obama, said Trump's out-of-the-blue threat to engulf Pyongyang in "fire and fury" indicated the U.S. leader is ignoring a diplomatic code his predecessor had always kept in mind: to alert the allies to Washington's unveiling of new language or policy directions on North Korea.

"It does cause some strain when that doesn't happen," said Rhodes, a self-acknowledged Trump critic.